What's wrong in this way?

my @a = (0,0,0,0);

$a[$_-1]  = $_, print join(':',@a),"\n" for 1..@a;
$a[$_-1]  = $_, print join(':',@a),"\n" for 1..@a;
$a[@a-$_] = $_, print join(':',@a),"\n" for 1..@a;

__END__

STDOUT:
1:0:0:0
1:2:0:0
1:2:3:0
1:2:3:4
1:2:3:4
1:2:3:4
1:2:3:4
1:2:3:4
1:2:3:1
1:2:2:1
1:3:2:1
4:3:2:1
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