You seem to be confusing arrays and array references. A reference is actually a scalar that holds an address of something. To get the thing that it addresses, you have to dereference it somehow.

In the data structure you have, this is a reference:

$hash_ref->{devices}->{device}
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To get the array, wrap it in @{} like so:

my @array = @{ $hash_ref->{devices}->{device} }
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Have a look at perlreftut, and maybe References quick reference,