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

           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 well as an
       integer).  Additionally, there is the UV, which is simply an unsigned
       IV.

       Perl also uses two special typedefs, I32 and I16, which will always 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
$