from the little I know it is very rare that memory is actually released to the OS. I think that some memory can be reused by your perl program but I suspect it will not be released (maybe this depends on the OS?).

Waiting for other more expert answers you can consider to use a memory profiling program to test out your assumption: Devel::SizeMe and Devel::Size seems to be appriopriate choices.

You can also consider to not load at all the file in memory: File::Map does exactly this. See memory-map-files-instead-of-slurping-them.

L*

PS please add some <code> tags around your code

PPS see also Mini-Tutorial: Perl's Memory Management which suprised me with the sentence: You are more likely to see memory being released to the OS on Windows

PPPS Perl and Garbage Collection linking to Do subroutine variables get destroyed?