# this will be ok
$hash{1}{\@array}{1} = 42;

#this will be ok
$hash{1}{\@array}{1}{1} = 42;
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The latter doesn't work either (it just doesn't issue an error here, 'cos the script is dying after the first problem on line 13).

The reason is simply that if you assign 42 to $hash{\@array}, or $hash{1}{\@array}{1} for that matter, and then try to deference that value in the next statement, you're essentially trying to say

  "42"->{1} = 42
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which obviously doesn't make sense.