Be sure to use strict; and use warnings;—they will catch bugs in your code so you don't have to.

Yeah, but it doesn't actually help catching the bug the OP is struggling with, does it?

Bare filehandles ("FH") are discouraged. Use open(my $fh, "<", $tu), three-argument open.

Bollocks. There's nothing wrong with the filehandle or the open the OP used.