Better read the link brother Choroba gave you and use this code instead.

$HoA{a}[3]=“hello";
print $HoA{a}[3];
[download]

See perldsc for details about nested data.

HTH :)

update

symbolic refs don't seem work with nested data, the only way to make your code "work" is

 $d = eval "$b$c"

please note that you forgot the $ sigil in $b ='$a' ( or $b ="\$a" if you need double quotes)