while ( s{ ^ (.*?)
               (\s* \S+)_DEL
               (.*) $ }
           {$1$3\n$1$2$3}xmg )
  {}

##</code><code>##

sub del_selective
{
    my $n = shift;
    if ( $n =~ m/(.*?)(\s*\S+)_DEL(.*)/s )
    {
        my $s1 = $1.$3;
        my $s2 = $1.$2.$3;
        return ( del_selective( $s1 ),
                 del_selective( $s2 ) );
    }
    else
    {
        return ( $n );
    }
}