print "stage 0\n"; 
sleep 5; # memory usage now: 1800 Kb
$a='a' x 10000000;
$b='b';
print "stage 1\n"; 
sleep 5; # memory usage now: 21336 Kb
($a,$b)=($b,$a);
print "stage 2\n";
sleep 5; # memory usage now: 31104 Kb