It's all documented :)

$ perldoc guts
:
Variables
   Datatypes
       Perl has three typedefs that handle Perl's three main data type
+s:

           SV  Scalar Value
           AV  Array Value
           HV  Hash Value

       Each typedef has specific routines that manipulate the various 
+data
       types.

   What is an "IV"?
       Perl uses a special typedef IV which is a simple signed integer
+ type
       that is guaranteed to be large enough to hold a pointer (as wel
+l as an
       integer).  Additionally, there is the UV, which is simply an un
+signed
       IV.

       Perl also uses two special typedefs, I32 and I16, which will al
+ways be
       at least 32-bits and 16-bits long, respectively. (Again, there 
+are U32
       and U16, as well.)  They will usually be exactly 32 and 16 bits
+ long,
       but on Crays they will both be 64 bits.
:
:
etc etc
$
[download]

Next to guts you might want to take a look at xs and xstut.

I don't think your WTF?'s are appropriate.