Thanks for your reply. Unfortunately, a Perl module around XEvIE doesn't seem to exist yet, so I'm stuck with X11::Protocol. On the other hand, I've taken a look into the source code of icewm and I've seen that it uses XGrabKey to grab the key combinations it uses. Something like:

(52 being the keycode for the letter 'z').

The only obstacle I have to overcome is how to translate 'z' to 52 (in this example) but I think X11::Keyboard's KeysymToKeycode will fit this purpose nicely.