my ($b, $a) = ($a, $b)

##</code><code>##

$a = 100;
$b = 200;
my ($a, $b) = ($b, $a);
print $a, "\n"; #200
print $b, "\n"; #100
print $main::a, "\n";#100
print $main::b, "\n";#200

##</code><code>##

($b, $a) = ($a, $b)