As far as I am aware less.pm is the only joke module in the Perl core.

The Perl source code and the venerable Configure script are littered with witty comments and Tolkien quotes, mostly courtesy of Larry Wall (see "Source code" and "Configure" sections at Larry Wall wikiquote). For example:

• double value; /* or your money back! */
• echo 'Congratulations. You aren't running Eunice.'

Parrot had a "Larry Wall/Dan Sugalski" easter egg hidden in its cc compiler options:

cc -I/usr/local/include -D_LARGEFILE_SOURCE -D_FILE_OFFSET_BITS=64 -I/
+usr/include/gdbm -g -Dan_Sugalski -Larry -Wall -Wstrict-prototypes...
[download]

Provoked by merlyn, pjf once submitted a patch to P5P to make Perl play nethack when sort is called in scalar context. Doubt this patch was ever accepted however.

I wonder if maybe there are some easter eggs hidden in perl?

I doubt it, but if so, they're hidden in plain sight: Perl source.

Despite its availability, nobody has ever accused the Perl source of being transparent and legible, though... ;)

Modules are a different story. Even core modules often have rich non-core histories before they're inducted into the Perl core distribution. Nevertheless, for the most part I think you would find them to be free from intentional surprises.