This appears to work ok:

$a ='string1';
@$a[0] = 'string2';
print @string1;
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What exactly is your problem?
Addition:oops - Note however that this will not work if you have

use strict;
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which is always a good idea.
I would examine why you want to write your code in this way as I am sure there is a 'better' way of doing it.
If you could show more of your code I/we could probably give you a better answer.