(radoulov was faster, shorter and probably better. Code below uses same principle, but very verbose.)

This program should do it (very verbose)

Code:

@a = qw/a c b a b d/;
%b = {};
@c = ();
foreach(@a) {
        if(!$b{$_}){
                push @c, $_;
                $b{$_}++;
        }
}
print "Output:\n" . join("\n",@c) . "\n";

Feel free to line it up...

Code:

@a = qw/a c b a b d/; %b = {}; @c = ();
foreach (@a) { push @c, $_ if !$b{$_}++; }
print "Output:\n" . join("\n",@c) . "\n";

Real perl guru's can probably make it even more crypting, reusing variables and special variables

Basically, you just use a hashtable to store the words, and each time check if the word is already in the hashtable (which is a O(1) action).