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perlfaq4 - Data Manipulation
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perlfaq4 - Data Manipulation ($Revision: 1.49 $, $Date: 1999/05/23 20:37:49 $)
The section of the FAQ answers questions related to the manipulation
of data as numbers, dates, strings, arrays, hashes, and miscellaneous
data issues.
The infinite set that a mathematician thinks of as the real numbers can
only be approximated on a computer, since the computer only has a finite
number of bits to store an infinite number of, um, numbers.
Internally, your computer represents floating-point numbers in binary.
Floating-point numbers read in from a file or appearing as literals
in your program are converted from their decimal floating-point
representation (eg, 19.95) to an internal binary representation.
However, 19.95 can't be precisely represented as a binary
floating-point number, just like 1/3 can't be exactly represented as a
decimal floating-point number. The computer's binary representation
of 19.95, therefore, isn't exactly 19.95.
When a floating-point number gets printed, the binary floating-point
representation is converted back to decimal. These decimal numbers
are displayed in either the format you specify with printf(), or the
current output format for numbers. (See $# in the perlvar manpage if you use
print. $# has a different default value in Perl5 than it did in
Perl4. Changing $# yourself is deprecated.)
This affects all computer languages that represent decimal
floating-point numbers in binary, not just Perl. Perl provides
arbitrary-precision decimal numbers with the Math::BigFloat module
(part of the standard Perl distribution), but mathematical operations
are consequently slower.
To get rid of the superfluous digits, just use a format (eg,
printf("%.2f", 19.95)) to get the required precision.
See Floating-point Arithmetic in the perlop manpage.
Perl only understands octal and hex numbers as such when they occur
as literals in your program. If they are read in from somewhere and
assigned, no automatic conversion takes place. You must explicitly
use oct() or hex() if you want the values converted. oct() interprets
both hex (``0x350'') numbers and octal ones (``0350'' or even without the
leading ``0'', like ``377''), while hex() only converts hexadecimal ones,
with or without a leading ``0x'', like ``0x255'', ``3A'', ``ff'', or ``deadbeef''.
This problem shows up most often when people try using chmod(), mkdir(),
umask(), or sysopen(), which all want permissions in octal.
chmod(644, $file); # WRONG -- perl -w catches this
chmod(0644, $file); # right
Remember that int() merely truncates toward 0. For rounding to a
certain number of digits, sprintf() or printf() is usually the easiest
route.
printf("%.3f", 3.1415926535); # prints 3.142
The POSIX module (part of the standard Perl distribution) implements
ceil(), floor(), and a number of other mathematical and trigonometric
functions.
use POSIX;
$ceil = ceil(3.5); # 4
$floor = floor(3.5); # 3
In 5.000 to 5.003 perls, trigonometry was done in the Math::Complex
module. With 5.004, the Math::Trig module (part of the standard Perl
distribution) implements the trigonometric functions. Internally it
uses the Math::Complex module and some functions can break out from
the real axis into the complex plane, for example the inverse sine of
2.
Rounding in financial applications can have serious implications, and
the rounding method used should be specified precisely. In these
cases, it probably pays not to trust whichever system rounding is
being used by Perl, but to instead implement the rounding function you
need yourself.
To see why, notice how you'll still have an issue on half-way-point
alternation:
for ($i = 0; $i < 1.01; $i += 0.05) { printf "%.1f ",$i}
0.0 0.1 0.1 0.2 0.2 0.2 0.3 0.3 0.4 0.4 0.5 0.5 0.6 0.7 0.7
0.8 0.8 0.9 0.9 1.0 1.0
Don't blame Perl. It's the same as in C. IEEE says we have to do this.
Perl numbers whose absolute values are integers under 2**31 (on 32 bit
machines) will work pretty much like mathematical integers. Other numbers
are not guaranteed.
To turn a string of 1s and 0s like 10110110 into a scalar containing
its binary value, use the pack() and unpack() functions (documented in
pack in the perlfunc manpage and unpack in the perlfunc manpage):
$decimal = unpack('c', pack('B8', '10110110'));
This packs the string 10110110 into an eight bit binary structure.
This is then unpacked as a character, which returns its ordinal value.
This does the same thing:
$decimal = ord(pack('B8', '10110110'));
Here's an example of going the other way:
$binary_string = unpack('B*', "\x29");
The behavior of binary arithmetic operators depends on whether they're
used on numbers or strings. The operators treat a string as a series
of bits and work with that (the string "3" is the bit pattern
00110011). The operators work with the binary form of a number
(the number 3 is treated as the bit pattern 00000011).
So, saying 11 & 3 performs the ``and'' operation on numbers (yielding
1). Saying "11" & "3" performs the ``and'' operation on strings
(yielding "1").
Most problems with & and | arise because the programmer thinks
they have a number but really it's a string. The rest arise because
the programmer says:
if ("\020\020" & "\101\101") {
# ...
}
but a string consisting of two null bytes (the result of "\020\020"
& "\101\101") is not a false value in Perl. You need:
if ( ("\020\020" & "\101\101") !~ /[^\000]/) {
# ...
}
Use the Math::Matrix or Math::MatrixReal modules (available from CPAN)
or the PDL extension (also available from CPAN).
To call a function on each element in an array, and collect the
results, use:
@results = map { my_func($_) } @array;
For example:
@triple = map { 3 * $_ } @single;
To call a function on each element of an array, but ignore the
results:
foreach $iterator (@array) {
some_func($iterator);
}
To call a function on each integer in a (small) range, you can use:
@results = map { some_func($_) } (5 .. 25);
but you should be aware that the .. operator creates an array of
all integers in the range. This can take a lot of memory for large
ranges. Instead use:
@results = ();
for ($i=5; $i < 500_005; $i++) {
push(@results, some_func($i));
}
This situation has been fixed in Perl5.005. Use of .. in a for
loop will iterate over the range, without creating the entire range.
for my $i (5 .. 500_005) {
push(@results, some_func($i));
}
will not create a list of 500,000 integers.
Get the http://www.perl.com/CPAN/modules/by-module/Roman module.
If you're using a version of Perl before 5.004, you must call srand
once at the start of your program to seed the random number generator.
5.004 and later automatically call srand at the beginning. Don't
call srand more than once--you make your numbers less random, rather
than more.
Computers are good at being predictable and bad at being random
(despite appearances caused by bugs in your programs :-).
http://www.perl.com/CPAN/doc/FMTEYEWTK/random , courtesy of Tom
Phoenix, talks more about this. John von Neumann said, ``Anyone who
attempts to generate random numbers by deterministic means is, of
course, living in a state of sin.''
If you want numbers that are more random than rand with srand
provides, you should also check out the Math::TrulyRandom module from
CPAN. It uses the imperfections in your system's timer to generate
random numbers, but this takes quite a while. If you want a better
pseudorandom generator than comes with your operating system, look at
``Numerical Recipes in C'' at http://www.nr.com/ .
The day of the year is in the array returned by localtime() (see
localtime in the perlfunc manpage):
$day_of_year = (localtime(time()))[7];
or more legibly (in 5.004 or higher):
use Time::localtime;
$day_of_year = localtime(time())->yday;
You can find the week of the year by dividing this by 7:
$week_of_year = int($day_of_year / 7);
Of course, this believes that weeks start at zero. The Date::Calc
module from CPAN has a lot of date calculation functions, including
day of the year, week of the year, and so on. Note that not
all businesses consider ``week 1'' to be the same; for example,
American businesses often consider the first week with a Monday
in it to be Work Week #1, despite ISO 8601, which considers
WW1 to be the first week with a Thursday in it.
Use the following simple functions:
sub get_century {
return int((((localtime(shift || time))[5] + 1999))/100);
}
sub get_millennium {
return 1+int((((localtime(shift || time))[5] + 1899))/1000);
}
On some systems, you'll find that the POSIX module's strftime() function
has been extended in a non-standard way to use a %C format, which they
sometimes claim is the ``century''. It isn't, because on most such systems,
this is only the first two digits of the four-digit year, and thus cannot
be used to reliably determine the current century or millennium.
If you're storing your dates as epoch seconds then simply subtract one
from the other. If you've got a structured date (distinct year, day,
month, hour, minute, seconds values), then for reasons of accessibility,
simplicity, and efficiency, merely use either timelocal or timegm (from
the Time::Local module in the standard distribution) to reduce structured
dates to epoch seconds. However, if you don't know the precise format of
your dates, then you should probably use either of the Date::Manip and
Date::Calc modules from CPAN before you go hacking up your own parsing
routine to handle arbitrary date formats.
If it's a regular enough string that it always has the same format,
you can split it up and pass the parts to timelocal in the standard
Time::Local module. Otherwise, you should look into the Date::Calc
and Date::Manip modules from CPAN.
Use the Time::JulianDay module (part of the Time-modules bundle
available from CPAN.)
Before you immerse yourself too deeply in this, be sure to verify that it
is the Julian Day you really want. Are you really just interested in
a way of getting serial days so that they can do date arithmetic? If you
are interested in performing date arithmetic, this can be done using
either Date::Manip or Date::Calc, without converting to Julian Day first.
There is too much confusion on this issue to cover in this FAQ, but the
term is applied (correctly) to a calendar now supplanted by the Gregorian
Calendar, with the Julian Calendar failing to adjust properly for leap
years on centennial years (among other annoyances). The term is also used
(incorrectly) to mean: [1] days in the Gregorian Calendar; and [2] days
since a particular starting time or `epoch', usually 1970 in the Unix
world and 1980 in the MS-DOS/Windows world. If you find that it is not
the first meaning that you really want, then check out the Date::Manip
and Date::Calc modules. (Thanks to David Cassell for most of this text.)
The time() function returns the current time in seconds since the
epoch. Take twenty-four hours off that:
$yesterday = time() - ( 24 * 60 * 60 );
Then you can pass this to localtime() and get the individual year,
month, day, hour, minute, seconds values.
Note very carefully that the code above assumes that your days are
twenty-four hours each. For most people, there are two days a year
when they aren't: the switch to and from summer time throws this off.
A solution to this issue is offered by Russ Allbery.
sub yesterday {
my $now = defined $_[0] ? $_[0] : time;
my $then = $now - 60 * 60 * 24;
my $ndst = (localtime $now)[8] > 0;
my $tdst = (localtime $then)[8] > 0;
$then - ($tdst - $ndst) * 60 * 60;
}
# Should give you "this time yesterday" in seconds since epoch relative to
# the first argument or the current time if no argument is given and
# suitable for passing to localtime or whatever else you need to do with
# it. $ndst is whether we're currently in daylight savings time; $tdst is
# whether the point 24 hours ago was in daylight savings time. If $tdst
# and $ndst are the same, a boundary wasn't crossed, and the correction
# will subtract 0. If $tdst is 1 and $ndst is 0, subtract an hour more
# from yesterday's time since we gained an extra hour while going off
# daylight savings time. If $tdst is 0 and $ndst is 1, subtract a
# negative hour (add an hour) to yesterday's time since we lost an hour.
#
# All of this is because during those days when one switches off or onto
# DST, a "day" isn't 24 hours long; it's either 23 or 25.
#
# The explicit settings of $ndst and $tdst are necessary because localtime
# only says it returns the system tm struct, and the system tm struct at
# least on Solaris doesn't guarantee any particular positive value (like,
# say, 1) for isdst, just a positive value. And that value can
# potentially be negative, if DST information isn't available (this sub
# just treats those cases like no DST).
#
# Note that between 2am and 3am on the day after the time zone switches
# off daylight savings time, the exact hour of "yesterday" corresponding
# to the current hour is not clearly defined. Note also that if used
# between 2am and 3am the day after the change to daylight savings time,
# the result will be between 3am and 4am of the previous day; it's
# arguable whether this is correct.
#
# This sub does not attempt to deal with leap seconds (most things don't).
#
# Copyright relinquished 1999 by Russ Allbery <rra@stanford.edu>
# This code is in the public domain
Short answer: No, Perl does not have a Year 2000 problem. Yes, Perl is
Y2K compliant (whatever that means). The programmers you've hired to
use it, however, probably are not.
Long answer: The question belies a true understanding of the issue.
Perl is just as Y2K compliant as your pencil--no more, and no less.
Can you use your pencil to write a non-Y2K-compliant memo? Of course
you can. Is that the pencil's fault? Of course it isn't.
The date and time functions supplied with Perl (gmtime and localtime)
supply adequate information to determine the year well beyond 2000
(2038 is when trouble strikes for 32-bit machines). The year returned
by these functions when used in a list context is the year minus 1900.
For years between 1910 and 1999 this happens to be a 2-digit decimal
number. To avoid the year 2000 problem simply do not treat the year as
a 2-digit number. It isn't.
When gmtime() and localtime() are used in scalar context they return
a timestamp string that contains a fully-expanded year. For example,
$timestamp = gmtime(1005613200) sets $timestamp to ``Tue Nov 13 01:00:00
2001''. There's no year 2000 problem here.
That doesn't mean that Perl can't be used to create non-Y2K compliant
programs. It can. But so can your pencil. It's the fault of the user,
not the language. At the risk of inflaming the NRA: ``Perl doesn't
break Y2K, people do.'' See http://language.perl.com/news/y2k.html for
a longer exposition.
The answer to this question is usually a regular expression, perhaps
with auxiliary logic. See the more specific questions (numbers, mail
addresses, etc.) for details.
It depends just what you mean by ``escape''. URL escapes are dealt
with in the perlfaq9 manpage. Shell escapes with the backslash (\)
character are removed with
s/\\(.)/$1/g;
This won't expand "\n" or "\t" or any other special escapes.
To turn "abbcccd" into "abccd":
s/(.)\1/$1/g; # add /s to include newlines
Here's a solution that turns ``abbcccd'' to ``abcd'':
y///cs; # y == tr, but shorter :-)
This is documented in the perlref manpage. In general, this is fraught with
quoting and readability problems, but it is possible. To interpolate
a subroutine call (in list context) into a string:
print "My sub returned @{[mysub(1,2,3)]} that time.\n";
If you prefer scalar context, similar chicanery is also useful for
arbitrary expressions:
print "That yields ${\($n + 5)} widgets\n";
Version 5.004 of Perl had a bug that gave list context to the
expression in ${...}, but this is fixed in version 5.005.
See also ``How can I expand variables in text strings?'' in this
section of the FAQ.
This isn't something that can be done in one regular expression, no
matter how complicated. To find something between two single
characters, a pattern like /x([^x]*)x/ will get the intervening
bits in $1. For multiple ones, then something more like
/alpha(.*?)omega/ would be needed. But none of these deals with
nested patterns, nor can they. For that you'll have to write a
parser.
If you are serious about writing a parser, there are a number of
modules or oddities that will make your life a lot easier. There are
the CPAN modules Parse::RecDescent, Parse::Yapp, and Text::Balanced;
and the byacc program.
One simple destructive, inside-out approach that you might try is to
pull out the smallest nesting parts one at a time:
while (s/BEGIN((?:(?!BEGIN)(?!END).)*)END//gs) {
# do something with $1
}
A more complicated and sneaky approach is to make Perl's regular
expression engine do it for you. This is courtesy Dean Inada, and
rather has the nature of an Obfuscated Perl Contest entry, but it
really does work:
# $_ contains the string to parse
# BEGIN and END are the opening and closing markers for the
# nested text.
@( = ('(','');
@) = (')','');
($re=$_)=~s/((BEGIN)|(END)|.)/$)[!$3]\Q$1\E$([!$2]/gs;
@$ = (eval{/$re/},$@!~/unmatched/);
print join("\n",@$[0..$#$]) if( $$[-1] );
Use reverse() in scalar context, as documented in
reverse in the perlfunc manpage.
$reversed = reverse $string;
You can do it yourself:
1 while $string =~ s/\t+/' ' x (length($&) * 8 - length($`) % 8)/e;
Or you can just use the Text::Tabs module (part of the standard Perl
distribution).
use Text::Tabs;
@expanded_lines = expand(@lines_with_tabs);
Use Text::Wrap (part of the standard Perl distribution):
use Text::Wrap;
print wrap("\t", ' ', @paragraphs);
The paragraphs you give to Text::Wrap should not contain embedded
newlines. Text::Wrap doesn't justify the lines (flush-right).
There are many ways. If you just want to grab a copy, use
substr():
$first_byte = substr($a, 0, 1);
If you want to modify part of a string, the simplest way is often to
use substr() as an lvalue:
substr($a, 0, 3) = "Tom";
Although those with a pattern matching kind of thought process will
likely prefer
$a =~ s/^.../Tom/;
You have to keep track of N yourself. For example, let's say you want
to change the fifth occurrence of "whoever" or "whomever" into
"whosoever" or "whomsoever", case insensitively. These
all assume that $_ contains the string to be altered.
$count = 0;
s{((whom?)ever)}{
++$count == 5 # is it the 5th?
? "${2}soever" # yes, swap
: $1 # renege and leave it there
}ige;
In the more general case, you can use the /g modifier in a while
loop, keeping count of matches.
$WANT = 3;
$count = 0;
$_ = "One fish two fish red fish blue fish";
while (/(\w+)\s+fish\b/gi) {
if (++$count == $WANT) {
print "The third fish is a $1 one.\n";
}
}
That prints out: "The third fish is a red one." You can also use a
repetition count and repeated pattern like this:
/(?:\w+\s+fish\s+){2}(\w+)\s+fish/i;
There are a number of ways, with varying efficiency. If you want a
count of a certain single character (X) within a string, you can use the
tr/// function like so:
$string = "ThisXlineXhasXsomeXx'sXinXit";
$count = ($string =~ tr/X//);
print "There are $count X characters in the string";
This is fine if you are just looking for a single character. However,
if you are trying to count multiple character substrings within a
larger string, tr/// won't work. What you can do is wrap a while()
loop around a global pattern match. For example, let's count negative
integers:
$string = "-9 55 48 -2 23 -76 4 14 -44";
while ($string =~ /-\d+/g) { $count++ }
print "There are $count negative numbers in the string";
To make the first letter of each word upper case:
$line =~ s/\b(\w)/\U$1/g;
This has the strange effect of turning ``don't do it'' into ``Don'T
Do It''. Sometimes you might want this. Other times you might need a
more thorough solution (Suggested by brian d. foy):
$string =~ s/ (
(^\w) #at the beginning of the line
| # or
(\s\w) #preceded by whitespace
)
/\U$1/xg;
$string =~ /([\w']+)/\u\L$1/g;
To make the whole line upper case:
$line = uc($line);
To force each word to be lower case, with the first letter upper case:
$line =~ s/(\w+)/\u\L$1/g;
You can (and probably should) enable locale awareness of those
characters by placing a use locale pragma in your program.
See the perllocale manpage for endless details on locales.
This is sometimes referred to as putting something into ``title
case'', but that's not quite accurate. Consider the proper
capitalization of the movie Dr. Strangelove or: How I Learned to
Stop Worrying and Love the Bomb, for example.
Take the example case of trying to split a string that is comma-separated
into its different fields. (We'll pretend you said comma-separated, not
comma-delimited, which is different and almost never what you mean.) You
can't use split(/,/) because you shouldn't split if the comma is inside
quotes. For example, take a data line like this:
SAR001,"","Cimetrix, Inc","Bob Smith","CAM",N,8,1,0,7,"Error, Core Dumped"
Due to the restriction of the quotes, this is a fairly complex
problem. Thankfully, we have Jeffrey Friedl, author of a highly
recommended book on regular expressions, to handle these for us. He
suggests (assuming your string is contained in $text):
@new = ();
push(@new, $+) while $text =~ m{
"([^\"\\]*(?:\\.[^\"\\]*)*)",? # groups the phrase inside the quotes
| ([^,]+),?
| ,
}gx;
push(@new, undef) if substr($text,-1,1) eq ',';
If you want to represent quotation marks inside a
quotation-mark-delimited field, escape them with backslashes (eg,
"like \"this\"". Unescaping them is a task addressed earlier in
this section.
Alternatively, the Text::ParseWords module (part of the standard Perl
distribution) lets you say:
use Text::ParseWords;
@new = quotewords(",", 0, $text);
There's also a Text::CSV (Comma-Separated Values) module on CPAN.
Although the simplest approach would seem to be
$string =~ s/^\s*(.*?)\s*$/$1/;
not only is this unnecessarily slow and destructive, it also fails with
embedded newlines. It is much faster to do this operation in two steps:
$string =~ s/^\s+//;
$string =~ s/\s+$//;
Or more nicely written as:
for ($string) {
s/^\s+//;
s/\s+$//;
}
This idiom takes advantage of the foreach loop's aliasing
behavior to factor out common code. You can do this
on several strings at once, or arrays, or even the
values of a hash if you use a slice:
# trim whitespace in the scalar, the array,
# and all the values in the hash
foreach ($scalar, @array, @hash{keys %hash}) {
s/^\s+//;
s/\s+$//;
}
(This answer contributed by Uri Guttman, with kibitzing from
Bart Lateur.)
In the following examples, $pad_len is the length to which you wish
to pad the string, $text or $num contains the string to be padded,
and $pad_char contains the padding character. You can use a single
character string constant instead of the $pad_char variable if you
know what it is in advance. And in the same way you can use an integer in
place of $pad_len if you know the pad length in advance.
The simplest method uses the sprintf function. It can pad on the left
or right with blanks and on the left with zeroes and it will not
truncate the result. The pack function can only pad strings on the
right with blanks and it will truncate the result to a maximum length of
$pad_len.
# Left padding a string with blanks (no truncation):
$padded = sprintf("%${pad_len}s", $text);
# Right padding a string with blanks (no truncation):
$padded = sprintf("%-${pad_len}s", $text);
# Left padding a number with 0 (no truncation):
$padded = sprintf("%0${pad_len}d", $num);
# Right padding a string with blanks using pack (will truncate):
$padded = pack("A$pad_len",$text);
If you need to pad with a character other than blank or zero you can use
one of the following methods. They all generate a pad string with the
x operator and combine that with $text. These methods do
not truncate $text.
Left and right padding with any character, creating a new string:
$padded = $pad_char x ( $pad_len - length( $text ) ) . $text;
$padded = $text . $pad_char x ( $pad_len - length( $text ) );
Left and right padding with any character, modifying $text directly:
substr( $text, 0, 0 ) = $pad_char x ( $pad_len - length( $text ) );
$text .= $pad_char x ( $pad_len - length( $text ) );
Use substr() or unpack(), both documented in the perlfunc manpage.
If you prefer thinking in terms of columns instead of widths,
you can use this kind of thing:
# determine the unpack format needed to split Linux ps output
# arguments are cut columns
my $fmt = cut2fmt(8, 14, 20, 26, 30, 34, 41, 47, 59, 63, 67, 72);
sub cut2fmt {
my(@positions) = @_;
my $template = '';
my $lastpos = 1;
for my $place (@positions) {
$template .= "A" . ($place - $lastpos) . " ";
$lastpos = $place;
}
$template .= "A*";
return $template;
}
Use the standard Text::Soundex module distributed with Perl.
Before you do so, you may want to determine whether `soundex' is in
fact what you think it is. Knuth's soundex algorithm compresses words
into a small space, and so it does not necessarily distinguish between
two words which you might want to appear separately. For example, the
last names `Knuth' and `Kant' are both mapped to the soundex code K530.
If Text::Soundex does not do what you are looking for, you might want
to consider the String::Approx module available at CPAN.
Let's assume that you have a string like:
$text = 'this has a $foo in it and a $bar';
If those were both global variables, then this would
suffice:
$text =~ s/\$(\w+)/${$1}/g; # no /e needed
But since they are probably lexicals, or at least, they could
be, you'd have to do this:
$text =~ s/(\$\w+)/$1/eeg;
die if $@; # needed /ee, not /e
It's probably better in the general case to treat those
variables as entries in some special hash. For example:
%user_defs = (
foo => 23,
bar => 19,
);
$text =~ s/\$(\w+)/$user_defs{$1}/g;
See also ``How do I expand function calls in a string?'' in this section
of the FAQ.
The problem is that those double-quotes force stringification--
coercing numbers and references into strings--even when you
don't want them to be strings. Think of it this way: double-quote
expansion is used to produce new strings. If you already
have a string, why do you need more?
If you get used to writing odd things like these:
print "$var"; # BAD
$new = "$old"; # BAD
somefunc("$var"); # BAD
You'll be in trouble. Those should (in 99.8% of the cases) be
the simpler and more direct:
print $var;
$new = $old;
somefunc($var);
Otherwise, besides slowing you down, you're going to break code when
the thing in the scalar is actually neither a string nor a number, but
a reference:
func(\@array);
sub func {
my $aref = shift;
my $oref = "$aref"; # WRONG
}
You can also get into subtle problems on those few operations in Perl
that actually do care about the difference between a string and a
number, such as the magical ++ autoincrement operator or the
syscall() function.
Stringification also destroys arrays.
@lines = `command`;
print "@lines"; # WRONG - extra blanks
print @lines; # right
Check for these three things:
- There must be no space after the << part.
- There (probably) should be a semicolon at the end.
- You can't (easily) have any space in front of the tag.
If you want to indent the text in the here document, you
can do this:
# all in one
($VAR = <<HERE_TARGET) =~ s/^\s+//gm;
your text
goes here
HERE_TARGET
But the HERE_TARGET must still be flush against the margin.
If you want that indented also, you'll have to quote
in the indentation.
($quote = <<' FINIS') =~ s/^\s+//gm;
...we will have peace, when you and all your works have
perished--and the works of your dark master to whom you
would deliver us. You are a liar, Saruman, and a corrupter
of men's hearts. --Theoden in /usr/src/perl/taint.c
FINIS
$quote =~ s/\s*--/\n--/;
A nice general-purpose fixer-upper function for indented here documents
follows. It expects to be called with a here document as its argument.
It looks to see whether each line begins with a common substring, and
if so, strips that substring off. Otherwise, it takes the amount of leading
whitespace fo | 
