The canonical answer (that perl is able to sometimes avoid) is to use a temporary variable, like so:

my $temp;
$temp=$a;
$a=$b;
$b=$c;
$c=$temp;
[download]

In the above code, we save the value of $a to a temporary variable. Then we assign $a to $b - without worrying about clobbering the value of $a, because we have just saved it. And now that the value in $b has been copied to $a, we can safely assign into $b, etc. etc...

Update: Thanks to those below. Yes, it should assign $c back to $temp, not $a.