If you have an other way of achieving the result I need, I'd be happy to learn about it.
In general, using an XML parser is preferable to using regular expressions. All parsers require an investment in time to learn, and XML::Twig is a good choice: 
use warnings;
use strict;
use XML::Twig;

my $x = '<map>
    <section1>
        <parameter>Some text</parameter>
        <rstate>CHANGE_THIS</rstate>
    </section1>
    <section2>
        <subsection>
            <parameter>dont change this</parameter>
            <rstate>CHANGE_THIS</rstate>
        </subsection>
        <subsection>
            <parameter>dont change this</parameter>
            <rstate>DONT CHANGE THIS</rstate>
        </subsection>        
        <subsection>
            <parameter>dont change this</parameter>
            <rstate>CHANGE_THIS</rstate>
        </subsection>
    </section2>
    <section3>
        <parameter>dont change this</parameter>
        <rstate>DONT CHANGE THIS</rstate>
    </section3>
</map>
';

my $t = XML::Twig->new(
    twig_handlers => { rstate => \&rstate },
    pretty_print  => 'indented',
);
$t->parse($x);
$t->print();

sub rstate {
    my ($t, $rstate) = @_;
    my $text = $rstate->text();
    $text =~ s/CHANGE_THIS/CHANGED/;
    $rstate->set_text($text);
}

__END__


<map>
  <section1>
    <parameter>Some text</parameter>
    <rstate>CHANGED</rstate>
  </section1>
  <section2>
    <subsection>
      <parameter>dont change this</parameter>
      <rstate>CHANGED</rstate>
    </subsection>
    <subsection>
      <parameter>dont change this</parameter>
      <rstate>DONT CHANGE THIS</rstate>
    </subsection>
    <subsection>
      <parameter>dont change this</parameter>
      <rstate>CHANGED</rstate>
    </subsection>
  </section2>
  <section3>
    <parameter>dont change this</parameter>
    <rstate>DONT CHANGE THIS</rstate>
  </section3>
</map>

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In reply to Re: regex: Need help substituting by toolic
in thread regex: Need help substituting by the_perl

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