Your test script was very educational for me. I'd never seen use of the Benchmark module before. It looks like you've certainly gotten a fast method for adding in the thousands separators.

For those who need something a bit easier to follow, I'd like to add the following to this topic:

#!/usr/bin/perl
use Number::Format;
my $num = 12349329824342;
my $nf = new Number::Format(-thousands_sep => ',');  
my $separated_num = $nf->format_number($num);
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