What cciulla means is that rename either returns 1 (success) or 0 (failure). Therefore this line of code is actually assigning 1 or 0 to the variable $newname:

my $newname = rename ($file, $mrn);
[download]

You should probably be doing something like this:

rename($file, $mrn) || die "Could not rename $file to $mrn - $!\n";
[download]

-- vek --