You don't have to dereference the hashref if you want to use it. For example, instead of:

my %arg = %{$arg}; for (keys %arg) { ... }
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you can do:

for (keys %$arg) { ... }
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That's what references allow in the first place, use it. :)

As for your other question, perldoc -f ref:

die "No what I want!" unless (ref($arg) eq "HASH");
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