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:
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...
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.
<code> <a> <b> <big> <blockquote> <br /> <dd> <dl> <dt> <em> <font> <h1> <h2> <h3> <h4> <h5> <h6> <hr /> <i> <li> <nbsp> <ol> <p> <small> <strike> <strong> <sub> <sup> <table> <td> <th> <tr> <tt> <u> <ul>