See perlfaq5: How can I output my numbers with commas added?. In addition, that FAQ apparently used to include the following, contributed by Andrew Johnson, as well:

sub commify {
   my $input = shift;
   $input = reverse $input;
   $input =~ s/(\d\d\d)(?=\d)(?!\d*\.)/$1,/g;
   return reverse $input;
}
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