You should read more about this, you can solve it with what is called a RegEx as for Regular Expression:

let's assume you know how to read the lines into an array and iterate over it, all you need to do is substitute s/// the abc into xyz:

ex:

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.
[download]

Update: this could chew user abcd@abc.com, so here's a fix

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]
[download]

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