If your aiming for a cross-platform solution, I doubt that there is an easy answer to this.

If your aiming at the Windows platform, then you may be able to do something with the latest version of dada's Win32API and use it to call user32.dll:RegisterHotkey and/or possibly user32.dll:RegisterShellHookWindow. You would need to get the system to call your code when your chosen hotkey is pressed. The latest version (0.41) of Win32::API purports to allow you to call win32 native APIs that require a callback address, though I haven't used that functionality yet.

You would probably need to create a gui window to act as the windowproc for callback. You might look at Win32::GUI to do that. There are also many useful functions in the Win32::GUITest module that would be useful for giving your window the focus etc. if and when you can get the system to call you.