From the OP:

${@$a}[0]
[download]

I had to stare at that for a bit and then do a little experiment:

>perl -wMstrict -le
"my @ra = qw(a b c d);
 my $ar = \@ra;
 print ${  ra}[2];
 print ${ @ra}[2];
 print ${ $ar}[2];
 print ${@$ar}[2];
"
c
c
c
c
[download]

Sure enough, it works! I'm gobsmacked!

Needless to say, I've never seen the second and fourth forms of the array access expression, the ones using the  @ sigil apparently redundantly. These forms are certainly not widely used, but are they ever used? I.e., is there any situation in which the  @ sigil must be used in this way?

Update: Added $ that was MIA in ${@$a}[0].