A simplistic version is:

use warnings;
use strict;

for (qw($100000000.00 $100.00 $1000.00)) {
    my $value = reverse $_;

    $value =~ s/(\d{3})(?=\d)/$1,/g;
    $value = reverse $value;
    print "$_ => $value\n";
}
[download]

Prints:

$100000000.00 => $100,000,000.00
$100.00 => $100.00
$1000.00 => $1,000.00
[download]

However that (as you can see) doesn't obey the convention that $1000.00 does not get a comma.