use strict;   # or die
use warnings; # or die

my %hash = (
   cogito   => "ergo sum"   # I think, so I exist
);

if (exists $hash{absum}) {
    print "Absum exists.";
} else {
    print "Absum doesn't exist.\n";
    # which would explain it's name...
}

print "Keys in the hash: ", join(" :: ", keys %hash), "\n";

__END__
Absum doesn't exist.
Keys in the hash: cogito

##</code><code>##

use strict;
use warnings;

my %hash = (
   list1 => ["foo", "bar", "baz"]

);


print "list1: [- ", join(" :: ", @{ $hash{list1} || []}), " -]\n";
print "list2: [- ", join(" :: ", @{ $hash{list2} || []}), " -]\n";

__END__
list1: [- foo :: bar :: baz -]
list2: [-  -]