I think you're confusing the memory allocation model of other languages, such as C, which has either static or dynamic allocation of memory; which may be set statically at compile-time and may be extended later at run-time (in C, realloc() & friends).

As Perl is written in C it will apply some magic automatically to data structures when operated upon to have them extended to an appropriate size.

There are no arbitrary limits except the ones imposed by such factors as the memory available, underlying operating system, compiler et al.