The current Slackware64 13.37 uses a build script that applies a small patch to Configure, and it changes hints/linux.sh. Apparently, those two changes make perl use lib64 instead of plain lib.

Note that Slackware64 is a pure 64 bit distribution, it has no 32 bit parts, and so the lib64 directories appear to be nonsense. The only reason for their existance is that Slackware64 is prepared for a 32 bit multilib addon. The other commonly known Linux distributions are multilib out of the box.

I think your OS distribution has more patches applied to perl that cause the lib/lib64 mix. Perhaps it installs pure-perl modules below lib (they could be used by both 32 bit and 64 bit perls) and modules with XS parts below lib64 or lib32, depending on which perl (32 bit or 64 bit) you use.

Alexander