You don't need to go as far as reading the string into an array, but just for fun

I wouldn't exactly call this type of text processing fun, but if you really must, the way you've shown is far more complicated than it needs to be:

  my $str = 'nodes0005';
  my @chars = split //, $str;
  @chars[3,4] = ("A", "B");
  my $out = join "", @chars;
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