I still can't see an error in your approach, but simply adding to an empty hash looks simpler, is easier to maintain and likely more time efficient. :)

  DB<103>  %copyhash=( key1 => '1', key15 => '1', key140 => '1', );
 => ("key1", 1, "key15", 1, "key140", 1)

  DB<104> %muthash = ( key1 => '1', key15 => '3', key150 => '1', );
 => ("key1", 1, "key15", 3, "key150", 1)

  DB<105> $mergedhash{$_} += $muthash{$_} for keys %muthash 
 => ""

  DB<106> $mergedhash{$_} += $copyhash{$_} for keys %copyhash 
 => ""

  DB<107> \%mergedhash 
 => { key1 => 2, key140 => 1, key15 => 4, key150 => 1 }
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