Probably the easiest thing to do given your example is use a substring with substr()

my $str = 'nodes0005';
substr($str,3,2,'AB');
print $str;

#   .. produces:

nodAB005

#   as required.
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Here we're replacing 2 chars from position 3 (remember that the numbering starts at 0 so that 0=n, 1=o, 3=d etc) with the string 'AB'. (see the perl manual for full details on using substr())

It is possible to use a regex but that would probably be over-complicating matters for such a simple case.

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

my $str = 'nodes0005';
my @chars = split //, $str;  # Split the string into an array.
my $out = ''; # Define an output string
for (my $i=0; $i<@chars; $i++) {
  if ( $i==3 ) {
    $out .= 'A';    # Character 4, replace with 'A'
  } elsif ($i==4) {
    $out .= 'B';    # Character 5, replace with 'B'
  } else {
    $out .= $chars[$i]; # Neither 4 or 5 so just straight output it
  }
}
print $out;
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