I can confirm that MacPerl on the (pre OS-X) Macintosh uses "LWP/Simple.pm" as format for the keys to %INC.

A few more things have been patched in MacPerl, in order to increase portability, so even stuff like

    do "LWP/Simple.pm";
[download]

would work (for specific definitions of "work"). It's the same situation with require, and use, as these have been built on top of do.

The normal file path separator on the Mac is ":". By contrast to Windows, "/" normally would not work. I distinctly recall being involved in a discussion about all this on the MacPerl mailing list a few years ago, but I'm currently still unsuccessful in tracing it back in the archives.
Update: Found it!

I have no idea on the situation on that other even more special case, VMS.