sed/awk, instead of Perl

[Steve Bertrand]

I'm frequently having to modify/convert email addresses from one 
format/domain to another.

Usually, I slap together a quick Perl script to do this for me. I don't 
do it frequently enough to keep track which one of my scripts does this 
for me, so I'm continuously re-inventing the wheel.

Some of the time, I use sed/awk to do this, but that usually requires a 
few passes over a few files.

To put it plainly, can anyone, if it's possible, provide a single line 
sed/awk pipeline that can:

- read email addresses from a file in the format:

user.name TAB domain.tld

- convert it to:

user_name at example.com

- write it back to either a new file, the original file, or to STDOUT

Regards,

Steve