This technique is very useful

Lookahead and lookbehind are both very useful indeed, but I think your example is better solved with a sexeger:

$amount = reverse $amount;
$amount =~ s/(\d{3})/$1,/g;
$amount = reverse $amount;
[download]

Update: oops, I guess my example doesn't commafy a floating-point number, now does it? Oh well. At least maybe someone will learn about sexegers who didn't know about them. 8^)