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
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
];
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.)