Putting Two Consecutive Lines into One Line with Perl / AWK

Question

I have a data like below:

abcd
join abcd
efgh
join efgh

I want to join the two consecutive pair into one line. Resulting:

abcd join abcd
efgh join efgh

How can I do it in Perl/AWK?

Answer 1

$ sed 'N;s/\n/ /' input.txt
abcd join abcd
efgh join efgh

Answer 2

The simplest way is:

paste - - < FILE

This joins using a space instead of a tab:

paste -d" " - - < FILE