my %List;
$List{"a"}{"1"} = "yes";
$List{"a"}{"2"} = "no";
$List{"b"}{"1"} = "possibly";
$List{"b"}{"2"} = "never";

##</code><code>##

print "Before:\n";
foreach my $priKey (sort keys %List)
{
    print "$priKey\n";
}
if (exists $List{"c"}{"1"} )
{
    print "found a bad one\n";
}
print "After:\n";
foreach my $priKey (sort keys %List)
{
    print "$priKey\n";
}

##</code><code>##

Before:
a
b
After:
a
b
c