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

[download]
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
        ];

[download]

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.)

In reply to Re: nonempty false list by toolic
in thread nonempty false list by Anonymous Monk

Title:
Use:  <p> text here (a paragraph) </p>
and:  <code> code here </code>
to format your post, it's "PerlMonks-approved HTML":


  • Posts are HTML formatted. Put <p> </p> tags around your paragraphs. Put <code> </code> tags around your code and data!
  • Titles consisting of a single word are discouraged, and in most cases are disallowed outright.
  • Read Where should I post X? if you're not absolutely sure you're posting in the right place.
  • Please read these before you post! —
  • Posts may use any of the Perl Monks Approved HTML tags:
    a, abbr, b, big, blockquote, br, caption, center, col, colgroup, dd, del, details, div, dl, dt, em, font, h1, h2, h3, h4, h5, h6, hr, i, ins, li, ol, p, pre, readmore, small, span, spoiler, strike, strong, sub, summary, sup, table, tbody, td, tfoot, th, thead, tr, tt, u, ul, wbr
  • You may need to use entities for some characters, as follows. (Exception: Within code tags, you can put the characters literally.)
            For:     Use:
    & &amp;
    < &lt;
    > &gt;
    [ &#91;
    ] &#93;
  • Link using PerlMonks shortcuts! What shortcuts can I use for linking?
  • See Writeup Formatting Tips and other pages linked from there for more info.