Perl allows you to use the value of one scalar as the identifier for another. Rarely is it a good idea, but occasionally it is useful.

$var = 'foo';
$foo = 'bar';
print $var;  # Will print "foo"
print $$var; # Will print "bar"
[download]

Also, you can use curly braces around a variable when it might be confusing to the compiler what you mean.

$foo = 'foo';
print "$food";   # Will print nothing (no scalar $food is defined)
print "${foo}d"; # Will print 'food' as expected
[download]