I'm too rushed to install the library and test it right now, but WWW::Search::Yahoo was last updated April 21, 2007. So it is unlikely to have aged into no longer working. What version are you using? The current Yahoo version is 2.409.

To check the version of your installed library you can use this one-liner:

perl -e -MWWW::Search::Yahoo -e 'print $WWW::Search::Yahoo::VERSION'
[download]

If you are on a windows system you'll need to replace the ' with ".