A simple question about the Perl line

[Majian]

Hi ,all:

I have a problem about this :

cat test:
12
23
34
45
56
67
...

I want to become like this :
1223
3445
5667
...

That means the next line is after the above line ~


I thought it for  a long time , but I have no idea yet~~

Can someone help me ?

Thanks~~~~