Also, why do I have to declare the variables $pfx and $num outside the while loop for this to work properly?

I think that is because the regular expression code blocks create closures around $pfx and $num when the regex is compiled. This means that the assignments will always be to the $pfx and $num that were declared inside the while loop the first time through. I think doing

local our ($pfx, $num);

inside the while would work but I've not tested it.

I hope this is of use.

Cheers,

JohnGG