Works for me ..

$ perl -e 'print join("\n",@INC)' | xargs -I % find % -name Mailer.pm
find: ‘/usr/local/lib/x86_64-linux-gnu/perl/5.24.1’: No such file or d
+irectory
find: ‘/usr/local/share/perl/5.24.1’: No such file or directory
/usr/share/perl5/Mail/Mailer.pm
[download]

As has already been pointed out, all you've found is a Mail directory. That directory may or may not have a Mailer.pm in it.