~/perl/change$ cat change.txt
Hello I am Kelly. Hello I am David. Hello I am Mark.
~/perl/change$ perl -pe 's/David/Josh/i' change.txt
Hello I am Kelly. Hello I am Josh. Hello I am Mark.
~/perl/change$ perl -p -i -e 's/David/Josh/i' change.txt
~/perl/change$ cat change.txt
Hello I am Kelly. Hello I am Josh. Hello I am Mark.