The reason it doesn't work is that you use $media{bond} before you've finished defining %media. Run under strict, therefore, it fails with

Global symbol "%media" requires explicit package name
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You get that error for both the lines where you use $media{bond} within the definition of %media. But this works fine:

my %media = (        
  bond    => "USLetter:Plain:white:75",    
);
$media{letter} = $media{bond};    
$media{plain} = $media{bond};
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Hey! Another reason to use strict!

§ George Sherston