I'm also in the process of learning some perl internals. Besides perlguts, perlxs, the book "Extending and Embedding perl" is a good read. One thing I learnt is the module Devel::Peek, using it you can see some internals of perl variables, for example

# devel.pl
use Devel::Peek;
my $a = "test";
Dump $a;
my $b = 5; 
Dump $b;
my $c = "123";
Dump $c;
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will give you:

SV = PV(0x22548c) at 0x18244e8
  REFCNT = 1
  FLAGS = (PADBUSY,PADMY,POK,pPOK)
  PV = 0x22cadc "test"\0
  CUR = 4
  LEN = 5
SV = IV(0x1828e5c) at 0x182450c
  REFCNT = 1
  FLAGS = (PADBUSY,PADMY,IOK,pIOK)
  IV = 5
SV = PV(0x2254a4) at 0x18244dc
  REFCNT = 1
  FLAGS = (PADBUSY,PADMY,POK,pPOK)
  PV = 0x22b26c "123"\0
  CUR = 3
  LEN = 4
[download]

Besides showing the ref count, the magic flags, you can roughly see how the in-memory structure looks like.