The following is off the cuff, but it might work for you. Note that it also allows for negative numbers. Unlike Popcorn Dave's solution, I combine the test and the number extraction into one step. You may or may not like this. (and I just cleaned up the code)

#!/usr/bin/perl -l
use strict;
use warnings;

my @numbers = ( 23, '1,234', '2,2', '12,234,567', -1, '-12,234',
   '-23,23', '999,999', 0 );

foreach my $number (@numbers) {
   if (my @sections = $number =~ /^(-?\d{1,3})(?:,(\d\d\d))*$/) {
       print join '', grep {defined} @sections;
   }
   else {
       # not a valid number
   }
}
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Cheers,
Ovid

New address of my CGI Course.
Silence is Evil (feel free to copy and distribute widely - note copyright text)