use strict;
use XML::Parser;

my $parser = new XML::Parser(
	Handlers => {
		Start => sub {
			my ($expat,$tag,%attr) = @_;
			print $attr{'molecule_idref'}, "\n" if %attr and exists $attr{'molecule_idref'};
		}
	}
);

$parser->parse(\*DATA);

__DATA__
<root>
<foo molecule_idref="123">fgdfg</foo>
<bar molecule_idref="456">fgd
 <foo other="74" molecule_idref="789">fgdfg</foo>
fg</bar>
some text about how to write things containing molecule_idref="666".
<baz molecule_idref="987"/>
</root>