I think there might be a couple of problems with your code.

• You join the output with \ns but you did not chomp the input so you will get extra blank lines.
• Hash keys are unordered so the line order of the original file will be scrambled. This may or may not be an issue.

The grep solutions suggested might be better if line order is to be preserved. Something like (again, not tested).

...
my %seen = ();
print grep ! $seen{ $_ } ++, <$fh>;
...
[download]

Cheers,

JohnGG