use Data::Dumper;
my @x; 
push @x, ({a=>3,b=>4}) x 5 ; 
$x[3]->{b}=9; # I want to modify the Fourth element only 
print Dumper ( \@x);"

-- Output -- 
$VAR1 = [
          {
            'a' => 3,
            'b' => 9
          },
          $VAR1->[0],
          $VAR1->[0],
          $VAR1->[0],
          $VAR1->[0]
        ];