${$lHashRef}{'a'} is just the "canonical" version of  $$lHashRef{'a'}. In the former syntax,  $lHashRef can be replaced by any expression that evaluates to a hash reference. In the latter syntax, thorny issues of precedence arise if  $lHashRef is replaced by anything other than another scalar hash reference.

c:\@Work\Perl\monks>perl -wMstrict -le
"my $s = undef;
 my $t = 0;
 my $u = { qw(a aye  b bee) };
 ;;
 print ${ $s || $t || $u }{a};
"
aye
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