It's often about perceived performance, I believe.

Sure, it's a clarity question here. What does that have to do with avoiding loops? I'd take

$b{$_}=1 for @a;
[download]

over

@b{@a} = (1)x@a;
[download]

even though the other might be considered to have no loop. (It has three while the first has two.)

Actually, I'd take the following since it incorporates the my:

my %b = map { $_ => 1 } @a;
[download]