I've never used perl under MS-DOS, and I haven't used MS-DOS for 10+ years, but I do vaguely remember a similar situation with an early DOS port of REXX that effectively blocked access to extended memory. I don't recall whether that was EMS or XMS (or even what the difference is though I knew once).

I suspect that the problem is not that Perl is using all the extended memory, but rather that it is trampling on something that effectively blocks access to it.

I tried to think of some way to verify this suspicion, I even dug out a couple of old MS-DOS internals books that I keep for "thems's was the days" purposes, but nothing leaped off the index or contents pages that looked like it would help:(