my $john;
while (defined(local $_ = <DATA>)) {
  if (/^(john\d+)$/) {
    $john = $1;
  }

  elsif (/^(jacob \- \d\.0)$/) {
    if ($john) {
      print "$john - $1\n";
    } else {
      die "Jacob is not preceded by John!";
    }
    undef $john;
  }
}


__DATA__
bhgfsggdsgsg
--
john1
weruwearnwrnweuarar
jjafdaiuweifweofiuwe
jacob - 1.0
--
nfaslf23523525
john2
asfsjldf43tgre
john3
asbdfhskafbv3333v
sdfahh34ttg
sadfhk34t3wtg
sdfhk3gfwghhw3
jacob - 2.0