Use the perl debugger. Do perl -d <script>. Once in the debugger switch to the module:

Defaria:perl -de 0

Loading DB routines from perl5db.pl version 1.33
Editor support available.

Enter h or `h h' for help, or `man perldebug' for more help.

main::(-e:1):    0
  DB<1> use CGI;

  DB<2> f CGI
Choosing /usr/share/perl/5.14/CGI/Util.pm matching `CGI':
1     package CGI::Util;
2     
3:    use strict;
4:    use vars qw($VERSION @EXPORT_OK @ISA @A2E @E2A);
5:    require Exporter;
6:    @ISA = qw(Exporter);
7:    @EXPORT_OK = qw(rearrange rearrange_header make_attributes unesc
+ape escape 
8             expires ebcdic2ascii ascii2ebcdic);
9     
10:    $VERSION = '3.51';
  DB<3>
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You'll notice the line starting with "Choosing". That tells you the path to the module that perl actually loaded.