The hash %media hasn't been scoped yet, the interpreter executes the 'line' at the semicolon. Therefore %media doesn't exist until perl sees the semicolon. In your case, I would suggest breaking out the assignments into different instructions. Such as the following:

my %media = (
  bond => "USLetter:Plain:white:75"
);

$media{'letter'} = $media{'bond'};
$media{'plain'} = $media{'bond'};
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Though there's probably better ways to do it through looping etc, it all depends on what you need done.