I'm familiar with assigning to slices of a hash, and its use in this line:

@seen {$t, $h, $r, $e} = ();

was very clever. However, why did you use slices in these lines?

@seen {$n,} = ();
....
@seen {$o,} = ();
[download]

Aren't they equivalent to:

$seen{$n} = undef;
$seen{$o} = undef;
[download]

Or even just:

@seen {$n} = ();  # note the lack of comma after $n
....
@seen {$o} = ();
[download]

I guess its that trailing comma that looks odd to me.

-Blake
p.s. props on the @HIGH and @LOW generation... very sneaky!