@saw{ @arr } = ();  # The line I'm not understanding
[download]

Others said that this is a hash slice. Yeah, but how, and why?

It's context again. Consider:

{};                             # hash reference
[];                             # array reference
%hash;                          # named hash
$hash{'foo'} = 'bar';           # left-hand-side (LHS) of '=' in scala
+r context
@ary = (1,2,3);                 # LHS & RHS are in list context:
                                #    assign a list to an array named '
+ary'

$hash{'foo'} = @ary;            # LHS context(scalar) forces RHS scala
+r context:
                                #    $hash{'foo'} == 3 , i.e. scalar(@
+ary)

@hash{qw(foo bar quux)} = @ary; # LHS in list context, RHS in list con
+text:
                                #    $hash{'bar'} == 2; $hash{'quux'} 
+== 3
[download]

The prefixes ($,%,@,&) for identifiers enforce context. So, @hash{@ary} = @list uses the %hash in list context, for which use the (imho) hard to grasp idiom 'slice', for naming the whole LHS expression, has settled.

--shmem

update: replaced "an array slice" with "a hash slice". Of course I meant "hash slice", as per ikegami's comment below. Glitches the like often happen to me, whilst thinking ahead. But best was a friend who reported she once thought "seven", said "1" and wrote "4", all at the same time..

_($_=" "x(1<<5)."?\n".q·/)Oo.  G°\        /
                              /\_¯/(q    /
----------------------------  \__(m.====·.(_("always off the crowd"))."·
");sub _{s./.($e="'Itrs `mnsgdq Gdbj O`qkdq")=~y/"-y/#-z/;$e.e && print}