Merge arguments with a file side by side

This is the prefix saved in a file

wolf@linux:~$ cat prefix.txt 
A. 
B. 
C. 
wolf@linux:~$ 

I would like to merge this with another input (in verfical format)

This is the sample input AB_CD_EF, however it’s in a horizontal format.

So I used tr to change it to vertical format.

wolf@linux:~$ echo AB_CD_EF | tr _ 'n'
AB
CD
EF
wolf@linux:~$ 

How do I merge these 2 output to produce:

A. AB
B. CD
C. EF

I’ve been thinking to use paste, but it doesn’t work with stdin.

wolf@linux:~$ paste $(echo AB_CD_EF | tr _ 'n') prefix.txt
paste: AB: No such file or directory
wolf@linux:~$ 

This data is not in sequence, I’m not looking for solution such as echo {A..C}

If there’s better way to do this, please let me know.

echo AB_CD_EF | tr _ 'n' | paste -d  prefix.txt -