my $string = 'This';
my $number = 100;
my $scalar_ref = \$string; # Take a reference to $string.
$$scalar_ref = 'That'; # Modify the value of the scalar that
                       # $scalar_ref refers to.
print "$$scalar_ref = $string\n"; # That = That because we 
                                  # changed the value of the 
                                  # thing that $scalar_ref
                                  # points to, which happens to 
                                  # also be what $string holds.
$scalar_ref = \$number;  # Change what $scalar_ref points to
                         # doesn't change the referent values.
print "$$scalar_ref isn't $string, it's $number\n";