Search for "slices" inside perldata:

Entire arrays (and slices of arrays and hashes) are denoted by '@', which works much like the word "these" or "those" does in English, in that it indicates multiple values are expected.

           @days               # ($days[0], $days[1],... $days[n])
           @days[3,4,5]        # same as ($days[3],$days[4],$days[5])
           @days{'a','c'}      # same as ($days{'a'},$days{'c'})
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(end quoting)
So that given a call like foobar(foo => "a", bar =>"b"), It translates to

my ($foo, $bar) = @{ {foo => "a", bar => "b"} }{ 'foo','bar'}
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which is essentially the same as ($passed_values{'foo'},$passed_values{'bar'}) It's just unusual because we don't use the @variable{'a','b'} form terribly often.