SPUG: to change a file in different folders

[Sachin Chaturvedi]

i want to modify a file i a file, say output.txt whaich is present in different folders.
for eg...

there is a line in output.txt 
source ../setext.cmd 
and i want to change it to  
source $testfolder/../setext.cmd.

i can do from comand line as perl -pi -e 's/old/new/g' output.txt 

but since this is in different folders i want to change all the output.txt in one shot calling script which automatically changes 
the folder one by one calls the above command and work is done