Adding just a wee tiny bit to vroom's and Fastolfe's answers...

This ditty accepts a list of numbers for commification, like so:
    my($minC, $maxC, $medC) = commify($min, $max, $med);
or so:
    my @numsC = commify(@nums);

sub commify {
  my @output;
  for(@_){
    my $input = $_;
    $input = reverse $input;
    $input =~ s<(\d\d\d)(?=\d)(?!\d*\.)><$1,>g;
    $input = reverse $input;
    push @output, $input;
  }
  return @output;
}
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Originally posted as a Categorized Answer.