Try File::Spec, namely its rel2abs() function: 
$abs_path = File::Spec->rel2abs( $path ) ;
$abs_path = File::Spec->rel2abs( $path, $base ) ;

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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?.

In reply to Re: How to get a file's full path? by athomason
in thread How to get a file's full path? by Bog

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