To remove \n but not \n\n

Data

23. Lorem
A) he
B) ha
C) hu
c

2. Ipsun yes right to write something here?
A) Ok
B) No 
C) yes
b

292. Ipsun
A) Apex
B) hello
b

289. He
A) hu
B) Ha
C) He
The right answer is:
1) Ab
2) Abc
3) Ac
2

where the replacement should return to meet Quizlet.com's syntax requirements

23. Lorem A) he B) ha C) hu
c    
2. Ipsun yes right to write something here? A) Ok B) No C) yes 
b
292. Ipsun A) Apex B) hello
b
289. He A) hu B) Ha C) He The right answer is: 1) Ab 2) Abc 3) Ac
2

This removes all empty spaces, s/\n//g in Vim.

How can you remove all \n but not \n\n in Perl or Vim? I think there is a need for Look-ahead because you cannot replace before you know that there is no second \n.