while (<DATA>) {

s/abc/xyz/ig; # The way this is constructed: it takes
              # the value in the first set of / / and
              # replaces it with the value in the next
              # set.

##</code><code>##

s/\@abc\.com/\@xyz\.com/ig; 

}


__DATA__
SMTP-Address: [user@abc.com]
              [user-1@abc.com]
              [user-2@abc.com]

SMTP-Address: [anotheruser@abc.com]
              [anotheruser-1@abc.com]
              [anotheruser-2@abc.com]