use strict;
use warnings;
use XML::LibXML;

my $doc = XML::LibXML->new->parse_fh(\*DATA);

# Your real file:
# my $doc = XML::LibXML->new->parse_file('/sand/devel/sthapa/tmp/as.xcf-dist');

for my $rsc ( $doc->getDocumentElement->findnodes("//RESOURCE") )
{
    unless ( $rsc->hasAttribute("Id") )
    {
        $rsc->setAttribute("Id", "AUTO-ID");
    }
}

print $doc->serialize(1);

# Or to a file-
# $doc->toFile($filename);

__DATA__
<?xml version="1.0"?>
<CONFIGURATION>
  <APPLICATION_SERVER>
    <RESOURCE_LIST>
      <PLATFORM_INDEPENDENT>
       <RESOURCE Id="res.wacgen" Source="INTERNAL">$(res.appl)</RESOURCE>
       <RESOURCE Id="res.wacgenlib" Source="INTERNAL">$(res.appl)$(res.slash)all.4gm</RESOURCE>
       <RESOURCE Id="res.waclibext" Source="INTERNAL">4gs</RESOURCE>
       <RESOURCE Id="res.fcode" Source="INTERNAL">rt.2.10.01-1138</RESOURCE>
       <RESOURCE Id="res.frunext" Source="INTERNAL">ifx</RESOURCE>
       <RESOURCE Id="res.db_type" Source="INTERNAL">msv</RESOURCE>
       <RESOURCE Source="INTERNAL">OH HAI, I CAN HAZ ID?</RESOURCE>
      </PLATFORM_INDEPENDENT>
    </RESOURCE_LIST>
  </APPLICATION_SERVER>
</CONFIGURATION>