Find strings in a file and write to the same file

[sappleg]

Hi Guys,

I have a question. Hope you could help. I currently have a txt file, containing three lines of text, e.g.

Line_1. ABC
Line_2. DEF
Line_3. GHI

I want to open this file, search for these three lines in the file and have a new line added to the bottom of the same file as:

Line_sum. ABC, DEF, GHI.

Please note that I am opening and writing to the same file that I am using to search the three strings.

Any help is appreciated.
Thanks a lot.

 

[KevinADC]

what have you tried so far?

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- Kevin, perl coder unexceptional!