This is not exactly what you asked (it's not Perl), but here's a piece of shell abuse that does it the hard way:

for path in `perl -e 'print join "\n", @INC'`; do
find $path -name "*.pm"|perl -pe "s|$path/||;s|/|::|g;s/\.pm$//;"
done |uniq
[download]

CU
Robartes- demonstrating that a combination of shell and Perl can be even gnarlier that either one alone :)