I suspect that all three of the choices you mentioned (tk, wx, gtk) might be equally easy to deploy across different systems -- or equally hard, depending on your attitude/abilities (and the attitude/abilities of the people being deployed to) regarding the installation of non-core Perl modules.

Actually, that might not be exactly correct: Tk is definitely easy to port, and while I haven't tried it, wx is probably just as easy; gtk might be a problem on some systems. But another consideration is ease of (programmer) use: Tk tends to be really well documented, and it has been the most heavily used over the years (hence a broader base of community experience to draw on).