Try removing this line:

        my $filename = q{$_};
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q// (or q{} etc.) is just the single-quote operator; I used that instead of the more customary single quotes to avoid having to escape the single quote in the string itself (which I felt would only serve to complicate the issue).

See Quote and Quote like Operators and Quote Like Operators in perlop. Since this IS a single-quote, BTW, q{$_} (which is exactly the same as '$_') does not interpolate $_ into the string, and you end up passing a literal $_ to md5sum. And of course, it's unlikely that a file with that name exists.

Long story short, just remove that line, it was just there to provide an example filename to work with.