There is no "database". It's just a tree of files, and yes, it's self-contained because Perl doesn't use any metadata besides the directory tree and the filenames in it.

However, as hbo explained, modules that used compiled parts, such as Gtk2 has to, may not be compatible between different versions of Perl. You can't necessarily move them around like that (though it's not out of the question).

I'd think you can run the regular Perl debugger on Perl GUI programs; and of course, you can still use print to output diagnostic messages to the console the program was started from. In unixoid systems, there is nothing special about GUI applications.