B::Xref, like the other B modules, looks through the internal bytecode generated by perl, so thus reuses perl's parser
Oh I hate
that mischaracterization. Perl doesn't normally have anything to do with bytecode. When the perl compiler parses perl (between toke.c and perly.y) it just constructs a bunch of C structs in memory. That's it. Those structs are the OP codes people sometimes mention. They're named stuff like enter, leave, const, padsv, print, etc. Perl executes by following these things around and occasionally triggering a C routine or two.
The only reason people ever mention bytecode is when making some Java comparison or when attempting to use B::Bytecode. Perl's bytecode is just a serialization of the opcode tree. It's also not a particularly effective hack either - (it's never worked for me).
So please, don't go on about Perl's bytecode. It doesn't use any. Unless you really mean to force the issue but that doesn't count because it's just loading another module. It's not actually 'perl'