Putting Two Consecutive Lines into One Line with Perl / AWK

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

$ sed 'N;s/\n/ /' tst.list 

abcd join abcd
efgh join efgh

http://stackoverflow.com/questions/10220348/putting-two-consecutive-lines-into-one-line-with-perl-awk