In the  $x{ @y } expression, array  @y is being evaluated in scalar context, i.e., it yields the number of elements of the array. This number is then stringized as a key for the hash.

>perl -wMstrict -le
"my %hash = qw(one uno  two dos);
 my @y = 5 .. 8;
 ;;
 print 'scalar @y: ', scalar @y;
 print 'does not exist' if not exists $hash{ @y };
 ;;
 $hash{ '4' } = 'four';
 print 'does exist: ', qq{'$hash{ @y }'} if exists $hash{ @y };
"
scalar @y: 4
does not exist
does exist: 'four'
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