If you really want to use regex, you could do this:

my $filespec = "/usr/share/directory/piano_book.mp4";
my $relative = "." . $1 if $filespec =~ m{/usr/share(.*)};
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Or you could use split and then paste together the pieces you need.

But you'd most probably be better off with a specialized module such as Path::Tiny mentioned above or some other.