Perl's garbage collection is implemented through reference counting (everything has a reference count, when the reference count drops to 0 the 'object' is removed). This can be demonstrated with a simple lexical variable and Devel::Peek

use Devel::Peek;

my $foo;
Dump($foo);
{
  my $r = \$foo;
  Dump($foo);
}
Dump($foo);

__output__

SV = NULL(0x0) at 0x8107e34
  REFCNT = 1
  FLAGS = (PADBUSY,PADMY)
SV = NULL(0x0) at 0x8107e34
  REFCNT = 2
  FLAGS = (PADBUSY,PADMY)
SV = NULL(0x0) at 0x8107e34
  REFCNT = 1
  FLAGS = (PADBUSY,PADMY)
[download]

There we can see $foo has an initial REFCNT (reference count) of 1 created by the file-level lexical scope, it is incremented when $r created a reference to it, and then decremented when $r goes out of scope (garbage collected because it's enclosing scope exitted and it's REFCNT dropped to 0) and would be garbage-collected once the file-scoped exits (which in this case is when the program ends). See. Matts wonderful Proxy Objects article for a more thorough review of reference counting in perl.
HTH

_________
broquaint