I didn't want it to be skill test

my @uniq = grep !$seen{$_}++, @words;
preserves order whereas
@seen{@words} = (); my @uniq = keys %seen;
doesn't. It's a well known idiom, not a "skill test". It's even listed in the answer to perldoc -q unique. (See perlfaq4)

wanted to merge with >4 chars

grep length>4, LIST will filter out all items except those with >4 chars, so you end up with

my @long = grep !$seen{$_}++, grep length>4, @words;
[download]

or you can combine the two grep calls into one

my @long = grep length>4 && !$seen{$_}++, @words;
[download]