perl: fix installing modules from contrib

Commit 20d2a30f (Makefile: replace perl/Makefile.PL with simple make rules)
removed a target that allowed Makefiles from contrib/ to get the correct
install path. This introduces a new target for main Makefile and fixes
installation for Mediawiki module.

v2: Pass prefix as that can have influence as well, add single quotes
    for _SQ variant.

Signed-off-by: Christian Hesse <mail@eworm.de>