${@$a}[0]

##</code><code>##

>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