When diving into a large code base and trying to understand how it
works, one thing that would be very helpful to know is how control
gets to a particular point in the code. That is, for a given
subroutine, what does the stack look like when the subroutine is
executed?

This is similar to what dtrace on Solaris can do for system calls, but
I am only looking to instrument perl subroutines (or points in perl
code.)

Also, like dtrace, it would be nice to do this on a running
perl/mod_perl process. It's kinda like being able to attach the perl
debugger to a running process.

I realize I'm probably just dreaming right now, but I figured I'd at
least bring up the idea with other perl users.