%seen = (
    paul  => 1,
    jane  => 1,
    betty => 1,
);

##</code><code>##

print for keys %seen;
# will print paul, jane, betty (though not necessarily in this order, see note below)

##</code><code>##

$seen{paul} = 1;       # this does absolutely nothing! but...
++$seen{paul};         # paul's *value* is 2 now
print for keys %seen;  # yet this still prints the same list

##</code><code>##

@list = grep !$temp{$_}++, @list; # same thing, shorter.