while ( my $line = <DATA>){
  my( @a, @b );
  my ( $junk, $useful ) = split /\;/, $line;
  my @elements = $useful =~ m/
    (\d+      # some numbers
     (?:      # followed by (don't capture)
      \(.*?\) # other stuff (non-greedy) in parens
     )?       # (optionally)
    )         # end capture
      /smxg;

  for my $element ( @elements ){
    if ( $element =~ /\(.*\)/ ){
      push @b, $element;
    }
    else{
      push @a, $element;
    }
  }
  # do something with @a and @b
}

__DATA__
2302.1;2302, 2304(3-8, 92-99), 2305(2, 4-9)
231.1;2301, 2303, 2304(1, 2, 90, 91), 2305(1, 3), 2306, 2307, 2308