my %set = ( a => undef, b => undef );
$set{c} = undef;

foreach my $element ( keys %set ) {
    printf "element '$element' at %s\n", \$set{$element};
}
__END__
element 'c' at SCALAR(0x504ee0)
element 'a' at SCALAR(0x504290)
element 'b' at SCALAR(0x504410)