perl plugin: Reimplemented plugin_{,un}register() in plain Perl.

perl plugin: Reimplemented plugin_{,un}register() in plain Perl.

As this involves only Perl data structures, it makes more sense to write
the algorithms in plain Perl as well. I suppose the perl interpreter can
do a better job optimizing the code :-)

Signed-off-by: Sebastian Harl <sh@tokkee.org>