#!perl

use HTML::Parser;
use HTML::Stream;

# see HTML::Stream documentation for how to output to a scalar
$output = new HTML::Stream \*STDOUT;

# inside FIELDs, ignore stuff that looks like tags
$insidefield = 0;

my $p = HTML::Parser->new
    (
     xml_mode => 1,
     start_h =>
     [sub {
        my ($tagname, $attr,$text) = @_;
        if ($insidefield) {$output->text($text);}
        else {$output->tag($tagname, %$attr);}
        if ($tagname eq 'FIELD') {$insidefield = 1;}
      }, "tagname, attr,text"],
     text_h =>
     [sub {
        my ($text) = @_;
        $output->text($text);
      }, "dtext"],
     end_h =>
     [sub {
        my ($tagname, $text) = @_;
        if ($tagname eq 'FIELD') {$insidefield = 0;}
        if ($insidefield) {$output->text($text);}
        else {$output->tag("_$tagname");}
      }, "tagname, text"],
    );

$p->parse_file(\*DATA);

__END__
<TRANSACTION>
    <FIELDS>
        <FIELD KEY="user">name</FIELD>
        <FIELD KEY="passwordlt">pass<word</FIELD>
        <FIELD KEY="passwordamp">pass&word</FIELD>
        <FIELD KEY="passwordgt">pass>word</FIELD>
        <FIELD KEY="passwordalmost">pass&ampword</FIELD>
        <FIELD KEY="passwordampcorrect">pass&amp;word</FIELD>
        <FIELD KEY="passwordtag">pa<ssw>ord</FIELD>
        <FIELD KEY="operation_type">do_what</FIELD>
     </FIELDS>
</TRANSACTION>