my @list= qw(a b c a c);
my $w="kandabbbc";
while (my $a=shift(@list)) {
$w=~s/\Q$a//;
@list = grep {!/\Q$a/} @list;
}
print $w;
####
my %saw;
for (@list) {
next if $saw{$_}++;
# ...
}
####
my @list= qw(a b c a c);
my $re = join '|', @list;
####
my $re=join "", @list;
$re="[$re]";