If you have a filename, then you probably already have some "relative" path to that filename, even if we're making the assumption that you're working from the current directory.

In this case, File::Spec will do the job for you with its rel2abs function:

$abs_path = File::Spec->rel2abs($filename);
[download]

If you have a file but have no idea where it's located, you might want to use File::Find.