sub stripPOS {
  my $words = shift;
  # rip out any / plus following characters, up to the 
  # first space
  $words =~ s!/\S*!!g;
  return $words;
}
  $sentence =~
    s! \s \<NP\d*\> (.*?) \</NP\> \s ! stripPOS($1). '/NP' !egx;

##</code><code>##

<NP1> <NP2> The/D best/A one/N </NP>
  of/P <NP2> the/D Perl/N Monks/N </NP> </NP>