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my $len=length($my_astr);
$c1 = substr($my_astr, $p1 = int rand($len), 1);
$c2 = substr($my_astr, $p2 = int rand($len), 1);

$p1 = int rand(length($my_astr));
$p2 = int rand(length($my_astr));
substr($my_astr, $p1, 1, substr($my_astr, $p2, 1));
substr($my_astr, $p2, 1, substr($my_astr, $p1, 1));