Here's a more generic solution that I used to commafy not only integers, but also decimals.

{
    local $_ = $amount;
    /\./ ? s/(?<=\d)(?=(\d{3})+(?:\.))/,/g
         : s/(?<=\d)(?=(\d{3})+(?!\d))/,/g;
    $amount = $_;
}
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If you look under Q&A, you will find related topics on how to commafy a number. You should learn to use the Super Search feature of the Perl Monks site. A lot of the questions have been discussed in detail, you just have to search for them.