use strict;
use warnings;
my @list = qw(A B C);
my %hash;
keys %hash = @list;
for my $element ( qw(D A) ) {
    $hash{$element}++ unless exists $hash{$element};
}
my @new_list = keys %hash;
print "Keys are @new_list.\n";

# now insert @list
@hash{@list} = ();
@new_list = keys %hash;
print "Keys are @new_list.\n";