Devel::Peek is a very useful module for looking at the Perl internals. It's especially useful for debugging XS code. With Devel::Peek we can look at Perl variables' data structures. This code:

use Devel::Peek;
my $x = 'mod_perl rules';
Dump $x;

prints:

SV = PV(0x804c674) at 0x80571fc
  REFCNT = 1
  FLAGS = (PADBUSY,PADMY,POK,pPOK)
  PV = 0x805ce78 "mod_perl rules"\0
  CUR = 14
  LEN = 15

We can see that this variable is a scalar, whose reference count is 1 (there are no other variables pointing to it). Its value is the string "mod_perl rules", terminated by \0 (one more character is used for the string-terminating \0 character, which is handled behind the scenes, transparently to the user), whose length is 15 characters including the terminating \0 character. The data structure starts at 0x80571fc, and its string value is stored starting from the address 0x805ce78.

If you want to look at more complicated structures, such as a hash or an array, you should create references to them and pass the references to the Dump( ) function.

The Apache::Peek module is built for use with mod_perl's Devel::Peek, so you can use it to peek at mod_perl's code internals.

In Chapter 10 we showed a few examples where Devel::Peek and Apache::Peek have been found very useful. To learn about Perl variables' internals, refer to the perlguts manpage.