how to insert new line in text file after regex match?

sumathigokul has asked for the wisdom of the Perl Monks concerning the following question:

Perl Gurus...

Please suggest me some idea to write particular statement after certain lines of regex match...

(1)first line.
(2)second line.
(3)third line.
(4)fourth line.
(5)fifth line.
[download]

when it finds regex "first", then output need to be like this...

(1)first line.
(2)second line.
 yes, first word is found
(3)third line.
(4)fourth line.
(5)fifth line.
[download]

Give me some example codes which may guide me to write the required code for my requirement??