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input:          desired output:     
  file1:          file1:            
...
    line1           line1           
    line2           line2           
    line3           line3

input:          desired output:     
  file1:          file1:            
...
    line2           line2           
    line3           line3
                    opt=bar

$ perl -i -pe '$f||=s/opt=.*/opt=bar/; END { print "opt=bar\n" if !$f 
+}' file1 file2

#!/bin/bash

...

echo --- cat after ---
cat file1 file2

--- cat before ---
line1
...
line3
line1
line2

--- cat before ---
line1
...
line2
line3
opt=bar