Just some standard pointer first - dont' say "PERL". It's not an acronymn. 
 Standardly capitalized "Perl" when speaking of the language in general, 
and lc "perl" when talking about specific code or script. But never PERL. 
Thanks.  You can google the backstory.

Next - it is important to use warning/strict in nearly anything a one 
liner, but esp. if you're going to post it to Perl list.  You'll (or you 
*should*) hear it every time you post code - it's the easiest, cheapest 
way to avoid foolish mistakes.  Any stylistic etc comments below are from 
my understanding of the coding bible "Perl Best Practices"  (D. Conway - 
O'Reilly).

sub { 
$output = ''; 

chomp $_[0]; 
$delim = $_[1]; 
@input_fields = split /$delim/, $_[0]; 

# input the values into the array 
for($i=0; $i<$#input_fields; $i++) { 
if i$ == 2 
{ 
# perform the calculation 
$input_fields[$i] = $input_fields[$i-1] * $input_fields[$i-2]; 
} 
output = $output . $input_fields[$i] . $delim; 
} 

$output = $output . $input_fields[$#input_fields] . "\n"; 
return($output); 
} 

sub params are better
   my ($data_str, $delim) = @_;
    chomp($data_str);

you need parens around the "if" criteria 
if ( $i == 2 ) {

For loops for arrays are better done perlishly
# input the values into the array 
for($i=0; $i<$#input_fields; $i++) { 

would normally be:
# input the values into the array 
for my $fld ( @input_fields ) { 

But you appear to be talking a string of numbers and delims:
2,4,6

splitting it up, multipling the first 2 and saving it in the 3rd (so your 
output is 2x4:
2,4,8,

)  W/ the "== 2" there, the calc. is only going to happen once, regardles 
of string length, which is probably not what you wanted. If you want every 
3rd field "($i + 1) % 3" maybe - +1 as arrays are zero based, and mod will 
return zero so:
if ( not ($i + 1) % 3 ) {

There are many ways to do this beside your route, but it mostly should 
work.

a

-------------------
Andy Bach
Systems Mangler
Internet: andy_bach@[...].gov
Voice: (608) 261-5738 Fax: 264-5932

"When angry, count to four; when very angry, swear."
Mark Twain

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