A related problem that may well be the underlying motivation of the question: writing an integer by block of three digits separated by underscores. The custom is to start from the end because the goal is to make visible thousands, millions and so on. The easy way:

$_=5544443333111;
$_=reverse; s/(\d{3})/$1_/g; $_=reverse; s/^_//; 
print;
[download]

-- stefp