By your code, I assume that $filestamp gets set as a side effect of the filestamp function? This would be more easily followed if you returned the timestamp from the function and called it somewhat like.

One problem I notice right away is that you are using single quotes to your argument to filestamp which means it always tests the string $logfile rather than the variable contents. So its likely that $filestamp is always set to 0 or more likely undef.

Unless you are using perl 4 then the & is optional for functions which have the ()'s after them. (and optional in other cases also)

Also the second foreach doesn't appear to be needed since each pass through the outer foreach will only return 1 logfile name to be processed.