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

#   .. produces:

nodAB005

#   as required.

##</code><code>##

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;