I suspect your 1st line returns no because the slice is being evaluated in scalar context and it is returning the last element in the list, which is undef. I will attempt to search for documentation to support this hypothesis.

If you switch the 1 and the 2, you get a yes:

$ perl -e '%x=(1 => 2); if(@x{2, 1}) { print "yes\n" } else { print "n
+o\n" }'
yes
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When you do the hash slice, you are asking for key 1 (which exists) and key 2 (which does not exist):

use warnings;
use strict;
use Data::Dumper; 

my %x = (1 => 2);
my @y = @x{1, 2};
print Dumper(\@y);

__END__

$VAR1 = [
          2,
          undef
        ];
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Update: see also: Context tutorial

Update: from Scalar values (emphasis mine):

If you evaluate an array in scalar context, it returns the length of the array. (Note that this is not true of lists, which return the last value, like the C comma operator, nor of built-in functions, which return whatever they feel like returning.)