Just two quick comments before heading to bed...

@l_Data = <FILE>; uses up at least twice 25MB (plus overhead), since the entire file is placed on the stack beore being put into @l_Data. push(@l_Data, $_) while <FILE>; is probably much better since only one line is on the stack at any given time.

perl generally doesn't release memory to the Operating System, only to itself. perl can reuse freed memory (such as the memory used by @l_Data), but the process size doesn't shrink. That's why the second call to Test barely uses up any memory from the OS's perspective.