There are a range of variations on the commify theme. That faq version is probably the worst. It is inefficient as it backtracks to do the job and also chokes on things like $1000 or 'string 123456'. The regex in my sub is one of merlyns creations but it chokes on >3 numbers after the decimal point without handling them separately. The Perl cookbook version:

sub commify {
    my $text = reverse $_[0];
    $text =~ s/(\d\d\d)(?=\d)(?!\d*\.)/$1,/g;
    return scalar reverse $text;
}
[download]

Is more more fault tolerant than the one from the faq.

That faq version is probably the worst.

Agreed, in general, though there are a range of variations on the "good" theme too.

The FAQ version is short and doesn't rely on look-ahead. Consequently it is the easiest to copy if you don't care how it works -- and easiest (IMHO) to understand if you do. Short and sweet has significant advantages if you are new to Perl, and that's why it's a good answer to a FAQ.

I did notice that the cookbook version you quote also appears (more or less) in the FAQ now.