Ok, here's a layman explanation:

Perl is free to do whatever it thinks might help your program run faster, usually at the expense of using more memory. You can't make any assumptions about memory usage of a script. In this case, perl probably created a compile-time constant for that which it copies when you ask for its value.

If you are working with extremely large data, you can't toss it around in the usual manners without large spikes of memory usage. You need to consider something like File::Map and then pass around references to that scalar instead of passing the scalar around by value. If you describe more of your needs, we can suggest better techniques to avoid loading it all into memory at once.