$hr->{a} = "hello" ; # or
@$hr{a} = "hello"  ; # doesn't work!!!
$ar->[0] = "hello" ; # or
@$ar[0] = "hello" ;  # does WORK !!!!

print "value of hr = $ar->[a]\n" ;
print "value of hr = $hr->{a}\n" ;

# or

print "value of ar = ".@$ar[0]."\n" ;
print "value of hr = ".%$hr{a}."\n" ; # doesn't work!!