The internal implementation of arrays is very space efficient.

All used slots point to values (scalar, constant ...), but unused slots don't demand much space or even no space.

• exists tells you if a slot is used
• assigning undef will fill a slot with the value undef
• defined will also return undef if a slot isn't used.

There are only little use cases where using exists on arrays can be of help.

You don't want to meddle with such internals.