sub commas {
    my $num = shift;
    my $f = '';
    while ( my ($r, $m) = $num =~ m/([0-9]+)([0-9]{3})$/ ) {
        $f = $r ? ",$m$f" : "$m$f";
        $num = $r;
    }
    return "$num$f";
}

printf("%s\n",commas(12345678)); # 12,345,678