use strict("refs"); 
or simply
use strict;

##</code><code>##

$a = 666;
$b = 888;

$name = "a";
print $$name, "\n";
$name = "b";
print $$name, "\n";


##</code><code>##

$name = "a";
&$name;
$name = "b";
&$name;

sub a {
    print "in sub a\n";
}

sub b {
    print "in sub b\n";
}