my @c = do { my %tmp = map { $_ => 1 } @a; grep {$tmp{$_}} @b };

##</code><code>##

for (1..@foo/2) { @foo[$_-1,-$_] = @foo[-$_,$_-1]; }

##</code><code>##

@foo = map { pop(@foo) } 1..@foo;