There is a very good and readable discussion of this in chapter 20 of Advanced Perl Programming.

The only way to get much deeper into it starts with fetching a copy of the current perl version:

rsync -auvz rsync://ftp.linux.activestate.com/perl-current/ /usr/local
+/lib/perl/bleadperl/
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Then building a debugging-enabled perl with debugging symbols in it (note the make option, this cost me some hours to find out):

pushd /usr/local/lib/perl/bleadperl
make realclean
rm config.sh
./Configure -de -Dcc=gcc -Dusedevel -Doptimize=-g3
make OPTIMIZE=-g3
make test
popd
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And then starting up your favorite debugger with a breakpoint at run.c:Perl_runops_debug, e.g., in gdb:

break run.c:Perl_runops_debug
run -e '$foo = 42'
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If that is your idea of fun, of course ...

Christian Lemburg
Brainbench MVP for Perl
http://www.brainbench.com