Try File::Spec, namely its rel2abs() function:

$abs_path = File::Spec->rel2abs( $path ) ;
$abs_path = File::Spec->rel2abs( $path, $base ) ;
[download]

It's a builtin module (at least in 5.8.7) so installation shouldn't be a concern.

Update:
See also the extended discussion at Absolute pathnames from relative?.