Basename.pm is included in the standard distro and will be located somewhere in your @INC directories in a directory called File which is its root namespace.

Try this on the CLI:
perl -e 'print join("\n", @INC);'

And you will see that for example in my system it is here with /usr/share/perl/5.8.0 being in my @INC:

barrd:~# locate Basename.pm
/usr/share/perl/5.8.0/File/Basename.pm
[download]

Update: Added more text to make a bit more sense