First, print what your @INC contains exactly:
perl -e "print @INC;"
and before you actually change it, make sure that you use Perl's DBD::mysql the right way which would be via DBI - as described in the perldoc page of DBD::mysql (perldoc DBD::mysql)
You'll have to have DBI AND DBD::mysql installed. DBI is a so called "core module" which means
As Perl's DBI interface is more or less always used the same way (despite the SQL dialects spoken to the database), please check one of the many, many available tutorials, you'll easily find with Google.
There's also a nice book by O'Reilly's.