For integers, one can avoid reverse as well as the repeated substr derivation of the remaining string in the loop:

sub commafy_int
{
    my $n = shift;
    length($n) > 3 or return $n;
    my $l = length($n) - 3;
    my $i = ($l - 1) % 3 + 1;
    my $x = substr($n,0,$i) . ',';
    while ( $i < $l )
    {
        $x .= substr($n,$i,3) . ',';
        $i += 3;
    }
    $x . substr($n,$i)
}
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Not using reverse also makes it much easier to convert this to work on floats (i.e. strings containing a decimal separator, '.'). To do so, simply replace any length calls above with index($n.'.','.').