The biggest trap of all is forgetting to use warnings or use the -w switch; see the perllexwarn manpage and the perlrun manpage. The second biggest trap is not making your entire program runnable under use strict. The third biggest trap is not reading the list of changes in this version of Perl; see the perldelta manpage.

Awk Traps

Accustomed awk users should take special note of the following:

A Perl program executes only once, not once for each input line. You can do an implicit loop with -n or -p.
The English module, loaded via
```
    use English;
```
allows you to refer to special variables (like $/) with names (like $RS), as though they were in awk; see the perlvar manpage for details.
Semicolons are required after all simple statements in Perl (except at the end of a block). Newline is not a statement delimiter.
Curly brackets are required on ifs and whiles.
Variables begin with "$", "@" or "%" in Perl.
Arrays index from 0. Likewise string positions in substr() and index().
You have to decide whether your array has numeric or string indices.
Hash values do not spring into existence upon mere reference.
You have to decide whether you want to use string or numeric comparisons.
Reading an input line does not split it for you. You get to split it to an array yourself. And the split() operator has different arguments than awk's.
The current input line is normally in $_, not $0. It generally does not have the newline stripped. ($0 is the name of the program executed.) See the perlvar manpage.
$<digit> does not refer to fields--it refers to substrings matched by the last match pattern.
The print() statement does not add field and record separators unless you set $, and $\. You can set $OFS and $ORS if you're using the English module.
You must open your files before you print to them.
The range operator is "..", not comma. The comma operator works as in C.
The match operator is "=~", not "~". ("~" is the one's complement operator, as in C.)
The exponentiation operator is "**", not "^". "^" is the XOR operator, as in C. (You know, one could get the feeling that awk is basically incompatible with C.)
The concatenation operator is ".", not the null string. (Using the null string would render /pat/ /pat/ unparsable, because the third slash would be interpreted as a division operator--the tokenizer is in fact slightly context sensitive for operators like "/", "?", and ">". And in fact, "." itself can be the beginning of a number.)
The next, exit, and continue keywords work differently.

The following variables work differently:

      Awk       Perl
      ARGC      scalar @ARGV (compare with $#ARGV)
      ARGV[0]   $0
      FILENAME  $ARGV
      FNR       $. - something
      FS        (whatever you like)
      NF        $#Fld, or some such
      NR        $.
      OFMT      $#
      OFS       $,
      ORS       $\
      RLENGTH   length($&)
      RS        $/
      RSTART    length($`)
      SUBSEP    $;

You cannot set $RS to a pattern, only a string.
When in doubt, run the awk construct through a2p and see what it gives you.

C/C++ Traps

Cerebral C and C++ programmers should take note of the following:

Curly brackets are required on if's and while's.
You must use elsif rather than else if.
The break and continue keywords from C become in Perl last and next, respectively. Unlike in C, these do not work within a do { } while construct. See Loop Control in the perlsyn manpage.
There's no switch statement. (But it's easy to build one on the fly, see Basic BLOCKs and Switch Statements in the perlsyn manpage)
Variables begin with "$", "@" or "%" in Perl.
Comments begin with "#", not "/*" or "//". Perl may interpret C/C++ comments as division operators, unterminated regular expressions or the defined-or operator.
You can't take the address of anything, although a similar operator in Perl is the backslash, which creates a reference.
ARGV must be capitalized. $ARGV[0] is C's argv[1], and argv[0] ends up in $0.
System calls such as link(), unlink(), rename(), etc. return nonzero for success, not 0. (system(), however, returns zero for success.)
Signal handlers deal with signal names, not numbers. Use kill -l to find their names on your system.

Sed Traps

Seasoned sed programmers should take note of the following:

A Perl program executes only once, not once for each input line. You can do an implicit loop with -n or -p.
Backreferences in substitutions use "$" rather than "\".
The pattern matching metacharacters "(", ")", and "|" do not have backslashes in front.
The range operator is ..., rather than comma.

Shell Traps

Sharp shell programmers should take note of the following:

The backtick operator does variable interpolation without regard to the presence of single quotes in the command.
The backtick operator does no translation of the return value, unlike csh.
Shells (especially csh) do several levels of substitution on each command line. Perl does substitution in only certain constructs such as double quotes, backticks, angle brackets, and search patterns.
Shells interpret scripts a little bit at a time. Perl compiles the entire program before executing it (except for BEGIN blocks, which execute at compile time).
The arguments are available via @ARGV, not $1, $2, etc.
The environment is not automatically made available as separate scalar variables.
The shell's test uses "=", "!=", "<" etc for string comparisons and "-eq", "-ne", "-lt" etc for numeric comparisons. This is the reverse of Perl, which uses eq, ne, lt for string comparisons, and ==, != < etc for numeric comparisons.

Perl Traps

Practicing Perl Programmers should take note of the following:

Remember that many operations behave differently in a list context than they do in a scalar one. See the perldata manpage for details.
Avoid barewords if you can, especially all lowercase ones. You can't tell by just looking at it whether a bareword is a function or a string. By using quotes on strings and parentheses on function calls, you won't ever get them confused.
You cannot discern from mere inspection which builtins are unary operators (like chop() and chdir()) and which are list operators (like print() and unlink()). (Unless prototyped, user-defined subroutines can only be list operators, never unary ones.) See the perlop manpage and the perlsub manpage.
People have a hard time remembering that some functions default to $_, or @ARGV, or whatever, but that others which you might expect to do not.
The <FH> construct is not the name of the filehandle, it is a readline operation on that handle. The data read is assigned to $_ only if the file read is the sole condition in a while loop:
```
    while (<FH>)      { }
    while (defined($_ = <FH>)) { }..
    <FH>;  # data discarded!
```
Remember not to use = when you need =~; these two constructs are quite different:
```
    $x =  /foo/;
    $x =~ /foo/;
```
The do {} construct isn't a real loop that you can use loop control on.
Use my() for local variables whenever you can get away with it (but see the perlform manpage for where you can't). Using local() actually gives a local value to a global variable, which leaves you open to unforeseen side-effects of dynamic scoping.
If you localize an exported variable in a module, its exported value will not change. The local name becomes an alias to a new value but the external name is still an alias for the original.

Perl4 to Perl5 Traps

Practicing Perl4 Programmers should take note of the following Perl4-to-Perl5 specific traps.

They're crudely ordered according to the following list:

Discontinuance, Deprecation, and BugFix traps: Anything that's been fixed as a perl4 bug, removed as a perl4 feature or deprecated as a perl4 feature with the intent to encourage usage of some other perl5 feature.
Parsing Traps: Traps that appear to stem from the new parser.
Numerical Traps: Traps having to do with numerical or mathematical operators.
General data type traps: Traps involving perl standard data types.
Context Traps - scalar, list contexts: Traps related to context within lists, scalar statements/declarations.
Precedence Traps: Traps related to the precedence of parsing, evaluation, and execution of code.
General Regular Expression Traps using s///, etc.: Traps related to the use of pattern matching.
Subroutine, Signal, Sorting Traps: Traps related to the use of signals and signal handlers, general subroutines, and sorting, along with sorting subroutines.
OS Traps: OS-specific traps.
DBM Traps: Traps specific to the use of dbmopen(), and specific dbm implementations.
Unclassified Traps: Everything else.

If you find an example of a conversion trap that is not listed here, please submit it to <perlbug@perl.org> for inclusion. Also note that at least some of these can be caught with the use warnings pragma or the -w switch.

Discontinuance, Deprecation, and BugFix traps

Anything that has been discontinued, deprecated, or fixed as a bug from perl4.

Symbols starting with "_" no longer forced into main

Symbols starting with "_" are no longer forced into package main, except for $_ itself (and @_, etc.).

    package test;
    $_legacy = 1;

    package main;
    print "\$_legacy is ",$_legacy,"\n";

    # perl4 prints: $_legacy is 1
    # perl5 prints: $_legacy is

Double-colon valid package separator in variable name
Double-colon is now a valid package separator in a variable name. Thus these behave differently in perl4 vs. perl5, because the packages don't exist.
```
    $a=1;$b=2;$c=3;$var=4;
    print "$a::$b::$c ";
    print "$var::abc::xyz\n";
```
```
    # perl4 prints: 1::2::3 4::abc::xyz
    # perl5 prints: 3
```
Given that :: is now the preferred package delimiter, it is debatable whether this should be classed as a bug or not. (The older package delimiter, ' ,is used here)
```
    $x = 10;
    print "x=${'x}\n";
```
```
    # perl4 prints: x=10
    # perl5 prints: Can't find string terminator "'" anywhere before EOF
```
You can avoid this problem, and remain compatible with perl4, if you always explicitly include the package name:
```
    $x = 10;
    print "x=${main'x}\n";
```
Also see precedence traps, for parsing $:.

2nd and 3rd args to splice() are now in scalar context

The second and third arguments of splice() are now evaluated in scalar context (as the Camel says) rather than list context.

    sub sub1{return(0,2) }          # return a 2-element list
    sub sub2{ return(1,2,3)}        # return a 3-element list
    @a1 = ("a","b","c","d","e");
    @a2 = splice(@a1,&sub1,&sub2);
    print join(' ',@a2),"\n";

    # perl4 prints: a b
    # perl5 prints: c d e

Can't do goto into a block that is optimized away

You can't do a goto into a block that is optimized away. Darn.

    goto marker1;

    for(1){
    marker1:
        print "Here I is!\n";
    }

    # perl4 prints: Here I is!
    # perl5 errors: Can't "goto" into the middle of a foreach loop

Can't use whitespace as variable name or quote delimiter

It is no longer syntactically legal to use whitespace as the name of a variable, or as a delimiter for any kind of quote construct. Double darn.

    $a = ("foo bar");
    $b = q baz;
    print "a is $a, b is $b\n";

    # perl4 prints: a is foo bar, b is baz
    # perl5 errors: Bareword found where operator expected

while/if BLOCK BLOCK gone

The archaic while/if BLOCK BLOCK syntax is no longer supported.

    if { 1 } {
        print "True!";
    }
    else {
        print "False!";
    }

    # perl4 prints: True!
    # perl5 errors: syntax error at test.pl line 1, near "if {"

** binds tighter than unary minus
The ** operator now binds more tightly than unary minus. It was documented to work this way before, but didn't.
```
    print -4**2,"\n";
```
```
    # perl4 prints: 16
    # perl5 prints: -16
```
foreach changed when iterating over a list
The meaning of foreach{} has changed slightly when it is iterating over a list which is not an array. This used to assign the list to a temporary array, but no longer does so (for efficiency). This means that you'll now be iterating over the actual values, not over copies of the values. Modifications to the loop variable can change the original values.
```
    @list = ('ab','abc','bcd','def');
    foreach $var (grep(/ab/,@list)){
        $var = 1;
    }
    print (join(':',@list));
```
```
    # perl4 prints: ab:abc:bcd:def
    # perl5 prints: 1:1:bcd:def
```
To retain Perl4 semantics you need to assign your list explicitly to a temporary array and then iterate over that. For example, you might need to change
```
    foreach $var (grep(/ab/,@list)){
```
to
```
    foreach $var (@tmp = grep(/ab/,@list)){
```
Otherwise changing $var will clobber the values of @list. (This most often happens when you use $_ for the loop variable, and call subroutines in the loop that don't properly localize $_.)
split with no args behavior changed
split with no arguments now behaves like split ' ' (which doesn't return an initial null field if $_ starts with whitespace), it used to behave like split /\s+/ (which does).
```
    $_ = ' hi mom';
    print join(':', split);
```
```
    # perl4 prints: :hi:mom
    # perl5 prints: hi:mom
```
-e behavior fixed
Perl 4 would ignore any text which was attached to an -e switch, always taking the code snippet from the following arg. Additionally, it would silently accept an -e switch without a following arg. Both of these behaviors have been fixed.
```
    perl -e'print "attached to -e"' 'print "separate arg"'
```
```
    # perl4 prints: separate arg
    # perl5 prints: attached to -e
```
```
    perl -e
```
```
    # perl4 prints:
    # perl5 dies: No code specified for -e.
```
push returns number of elements in resulting list
In Perl 4 the return value of push was undocumented, but it was actually the last value being pushed onto the target list. In Perl 5 the return value of push is documented, but has changed, it is the number of elements in the resulting list.
```
    @x = ('existing');
    print push(@x, 'first new', 'second new');
```
```
    # perl4 prints: second new
    # perl5 prints: 3
```
Some error messages differ
Some error messages will be different.
split() honors subroutine args
In Perl 4, if in list context the delimiters to the first argument of split() were ??, the result would be placed in @_ as well as being returned. Perl 5 has more respect for your subroutine arguments.
Bugs removed
Some bugs may have been inadvertently removed. :-)

Parsing Traps

Perl4-to-Perl5 traps from having to do with parsing.

Space between . and = triggers syntax error

Note the space between . and =

    $string . = "more string";
    print $string;

    # perl4 prints: more string
    # perl5 prints: syntax error at - line 1, near ". ="

Better parsing in perl 5

Better parsing in perl 5

    sub foo {}
    &foo
    print("hello, world\n");

    # perl4 prints: hello, world
    # perl5 prints: syntax error

Function parsing

"if it looks like a function, it is a function" rule.

  print
    ($foo == 1) ? "is one\n" : "is zero\n";

    # perl4 prints: is zero
    # perl5 warns: "Useless use of a constant in void context" if using -w

String interpolation of $#array differs

String interpolation of the $#array construct differs when braces are to used around the name.

    @a = (1..3);
    print "${#a}";

    # perl4 prints: 2
    # perl5 fails with syntax error

    @ = (1..3);
    print "$#{a}";

    # perl4 prints: {a}
    # perl5 prints: 2

Perl guesses on map, grep followed by { if it starts BLOCK or hash ref
When perl sees map { (or grep {), it has to guess whether the { starts a BLOCK or a hash reference. If it guesses wrong, it will report a syntax error near the } and the missing (or unexpected) comma.

Use unary + before { on a hash reference, and unary + applied to the first thing in a BLOCK (after {), for perl to guess right all the time. (See map in the perlfunc manpage.)

Numerical Traps

Perl4-to-Perl5 traps having to do with numerical operators, operands, or output from same.

Formatted output and significant digits
Formatted output and significant digits. In general, Perl 5 tries to be more precise. For example, on a Solaris Sparc:
```
    print 7.373504 - 0, "\n";
    printf "%20.18f\n", 7.373504 - 0;
```
```
    # Perl4 prints:
    7.3750399999999996141
    7.375039999999999614
```
```
    # Perl5 prints:
    7.373504
    7.375039999999999614
```
Notice how the first result looks better in Perl 5.

Your results may vary, since your floating point formatting routines and even floating point format may be slightly different.
Auto-increment operator over signed int limit deleted
This specific item has been deleted. It demonstrated how the auto-increment operator would not catch when a number went over the signed int limit. Fixed in version 5.003_04. But always be wary when using large integers. If in doubt:
```
   use Math::BigInt;
```
Assignment of return values from numeric equality tests doesn't work
Assignment of return values from numeric equality tests does not work in perl5 when the test evaluates to false (0). Logical tests now return a null, instead of 0
```
    $p = ($test == 1);
    print $p,"\n";
```
```
    # perl4 prints: 0
    # perl5 prints:
```
Also see General Regular Expression Traps using s///, etc. for another example of this new feature...
Bitwise string ops
When bitwise operators which can operate upon either numbers or strings (& | ^ ~) are given only strings as arguments, perl4 would treat the operands as bitstrings so long as the program contained a call to the vec() function. perl5 treats the string operands as bitstrings. (See Bitwise String Operators in the perlop manpage for more details.)
```
    $fr
```