Two problems here:

• You didn't actually call open_file in your code snippet there, so it won't work no matter what, because you haven't yet opened the file.
• I don't think you can use complex expressions in a <> operator. You need simple expressions, so try this:

      my $fh = $self->{FH};
      while (defined($line = <$fh>)) {
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Just a note: I'm assuming that you posted here because you got an error when running your code. It's helpful to actually post that error to give others some idea of the problem you're having.