This is yet another case in the ongoing trial of perl's auto-vivification. What's happening there is that by creating a reference to the first element of @y you are forcing it into existence, however it will still be undefined as you have yet to assign anything to it e.g

use Data::Dumper;
$Data::Dumper::Terse  = 1;
$Data::Dumper::Indent = 1;

my @y;

print 'pre auto-vivify:    ', Dumper(\@y);
print 'reference to $y[0]: ', Dumper(\$y[0]);
print 'post auto-vivify:   ', Dumper(\@y);

__output__

pre auto-vivify:    []
reference to $y[0]: \undef
post auto-vivify:   [
  undef
]
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