How about:

% perl -le 'print for @INC'

  /usr/local/perllibs
  /usr/local/lib/perl5/5.6.1/i586-linux
  /usr/local/lib/perl5/5.6.1
  /usr/local/lib/perl5/site_perl/5.6.1/i586-linux
  /usr/local/lib/perl5/site_perl/5.6.1
  /usr/local/lib/perl5/site_perl
  .
[download]

At least that gives you a list to work from.... You can also examine the output of perldoc perllocal which will provide information about locally installed modules:

% perldoc -t perllocal | grep -A 8 Digest::MD5

  Tue Aug 28 15:29:48 2001: "Module" Digest::MD5

    *   "installed into: /usr/local/lib/perl5/site_perl/5.6.1"

    *   "LINKTYPE: dynamic"

    *   "VERSION: 2.16"

    *   "EXE_FILES: "
[download]

-Blake