CPAN (as always) is your friend. Look at the URI module, particularly the abs method.

$uri->abs( $base_uri )
This method returns an absolute URI reference. If $uri already is absolute, then a reference to it is simply returned. If the $uri is relative, then a new absolute URI is constructed by combining the $uri and the $base_uri, and returned.

--
<http://www.dave.org.uk>

"The first rule of Perl club is you do not talk about Perl club."
-- Chip Salzenberg