Fletch has already answered your question, but...

I want to manipulate the array in $myHash{"tres"}, and in order to prevent deferencing it all the time, I'll copy it into an actual array

Personally, i prefer to just copy the reference.

my $aref = $myHash{"tres"};
$aref->[0] = 'bla';
# etc.
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This also avoids copying the arrays. Dereferencing is a tiny bit slower than using an alias, but my guess is that it just won't be noticable, and using local() unless you really have to is just ugly IMO.