use Data::Dumper;

my @a = qw/a b c d e/;
my %a;
@a{@a} = (1) x @a;

print "--- \@a before ---\n", Dumper \@a;
print "--- \%a before ---\n", Dumper \%a;

undef @a{@a};
undef @a[1,2,3];

print "--- \@a after ---\n", Dumper \@a;
print "--- \%a after ---\n", Dumper \%a;