change requires with perl

Shell Programming and Scripting
1

Hi,

i need to change a file with perl having contents below, but it will not do any action if change is being already made.

file.txt
AAAAAA
BBBBBB
CCCCCC

output.txt (that required)

AAAAAA
#BBBBB
DDDDDD

CCCCCC

Thanks,

2

sed is easier...

sed 's/^BBBB/#&/'  file > newfile
3

Hi jim,

its so nice you see my post answer, but lets see there are two points, lets suppose i want to comment a particular line number with "#" and wants to add one line text on particular line number, for example i want to comment 45th line and wants to add another line after 90th line and if it is already done so it wont do anything how can i put this.

Thanks, pls advice.
bash