use XML::Twig;
undef $/; 
$x=<DATA>;
## i have assumed that dc:identifier as first child of rdf:Description.
$p=new XML::Twig(twig_roots=>{
	'rdf:Description'=>\&rdf},
	pretty_print => 'indented');
$p->parse($x);
sub rdf
{
	my ($a,$b)=@_;
	my $filename=$b->first_child_text;	
	open(FOUT,">$filename") or die("$filename not opened for writing\n");
	print FOUT q(<?xml version="1.0" encoding="UTF-8"?>); 
	print FOUT $b->sprint;
	close(FOUT);
}
__DATA__
<?xml version="1.0" encoding="UTF-8"?>
<ab:root>
<rdf:Description>
<dc:identifier>ab.xml</dc:identifier>
<dc:abc>abc</dc:abc>
</rdf:Description>
<rdf:Description>
<dc:identifier>bc.xml</dc:identifier>
<dc:abc>bcd</dc:abc>
</rdf:Description>
<rdf:Description>
<dc:identifier>ef.xml</dc:identifier>
<dc:abc>efg</dc:abc>
</rdf:Description>
</ab:root>