$ perl -wMstrict -MData::Dump -e'  my $x = $#{$$a{list}};  dd $a'
{ list => [] }
$ perl -wMstrict -MData::Dump -e'  my $x = $#{@$a{list}};  dd $a'
Can't use an undefined value as an ARRAY reference at -e line 1.

##</code><code>##

$ perl -MData::Dump -e' my %h = map {$_ => {v => "x$_"}} qw(a b c); dd @h{qw(a b c)}->{v};'
"xc"