I'm not sure what you intended with your code, but let's look at what Perl sees:

>perl -w -MO=Deparse tmp.pl
BEGIN { $^W = 1; }
sub BEGIN {
    require 5.01;
}
use strict 'refs';
BEGIN {
    $^H{'feature_say'} = q(1);
    $^H{'feature_state'} = q(1);
    $^H{'feature_switch'} = q(1);
}
my(@x) = ('x', 1);
my $y = \@x;
say $#@ x [1];
[download]

This is the interesting line:

say $#@ x [1];
[download]

So Perl doesn't see anything regarding @x there, but sees the variable $#@ (the last index of the @@ array, repeated [1] times. Any reference in numerical context evalues to a (large) number. This is what causes the repeated output of -1.