I am expecting some low-level hack here. :-)

The low level hacks are found here

I was hoping Devel::Peek might reveal something but it doesn't :)

$ perl -Mthreads -MDevel::Peek -e"my $locked=1; my $unlocked=1; Dump($
+unlocked); lock($locked); Dump($locked)"

SV = IV(0x98a490) at 0x98a494
  REFCNT = 1
  FLAGS = (PADMY,IOK,pIOK)
  IV = 1
SV = IV(0x3e8c10) at 0x3e8c14
  REFCNT = 1
  FLAGS = (PADMY,IOK,pIOK)
  IV = 1
[download]

http://cpansearch.perl.org/src/JDHEDDEN/threads-shared-1.33/shared.xs says

/* User-level locks:
   This structure is attached (using ext magic) to any shared SV that
[download]

so you might have luck using Variable::Magic to peek at the magic