Its because %media hasn't been declared completely yet. It would be like trying to do this:

my $var = "blah".substr($var, 0, 2);
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Just because part of it is declared, doesn't mean the whole thing is. If you must reuse the code, store it in an earlier variable:

my $temp = "USLetter:Plain:white:75";
my %media = (        
  bond    => $temp,    
  letter  => $temp,    
  plain   => $temp
);
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