my @arr = (4, 1, 6, 4, 3, 7);
my %saw;
@saw{ @arr } = ();  # The line I'm not understanding
my @new_arr = keys %saw;
print sort @new_arr;

##</code><code>##

my @arr = (4, 1, 6, 4, 3, 7);
my %saw = map{ $_ => undef } @arr;
my @new_arr = keys %saw;
print sort @new_arr;