How to replace some string in file

[email protected] ("Santosh Reddy") Thu, 5 Jun 2008 12:32:52 +0530

Newsgroups	perl.beginners
Message-ID	<[email protected]>

Hi All,

I have a file called a.txt which has the following lines

abcd
efgh
ijkl
abcd123
12abcd

I want to replace abcd with xyz.

how do i do this. thanks a lot in advance

-- 
Thanks & Regards,
Santosh Reddy.

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