c:\@Work\Perl\monks>perl -wMstrict -le
"my $filespec = '/usr/share/directory/piano_book.mp4';
 print qq{absolute: '$filespec'};
 ;;
 my $dir = qr{ / [^/]+ }xms;  # fix [^/]
 ;;
 my $n = 2;
 die qq{'$filespec' could not be made relative}
   unless $filespec =~ s{ \A (?: $dir){$n} }{.}xms;
 print qq{relative: '$filespec'};
"
absolute: '/usr/share/directory/piano_book.mp4'
relative: './directory/piano_book.mp4'