I believe this will get what you want:

use strict;
use warnings;

while ( <DATA> ) {
 chomp;
 my @planes = m'<M>((?:[^<]+|<(?!/M(?:\d+)?>)))</M\d+>'gi;
 my @terms = m'<M>((?:[^<]+|<(?!/M>)))</M>'gi;
 print "@planes\n";
 print "@terms\n";
}

__DATA__
America's first <M>swept-wing</M>, <M>multiengine jet</M> <M>bomber</M
+> was the <M>B-47 Stratojet</M200>, and the first <M>swept-wing fight
+er</M> was the <M>F-86 Sabre Jet</M201>. Both used new swept-wing dat
+a found in Germany after <M>World War II</M> and sent back to the Uni
+ted States by American scientists. This photograph, from <D>1951</D>,
+ was taken the first time the two flew together over <PL>Kansas</PL>.
[download]

-enlil