input:          desired output:     
  file1:          file1:            
    line1           line1           
    opt=foo         opt=bar             
    line3           line3           
  file2:          file2:            
    line1           line1           
    line2           line2           
    line3           line3           

##</code><code>##

input:          desired output:     
  file1:          file1:            
    line1           line1           
    line2           line2             
    line3           line3           
  file2:          file2:            
    line1           line1           
    line2           line2           
    line3           line3
                    opt=bar

##</code><code>##

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

##</code><code>##

#!/bin/bash

#echo -e "line1\nopt=foo\nline3" >file1   # case 1
echo -e "line1\nline2\nline3"   >file1    # case 2
echo -e "line1\nline2\nline3"   >file2

echo --- cat before ---
cat file1 file2

echo --- perl output to stdout ---
perl -i -pe '$f||=s/opt=.*/opt=bar/; $_.="opt=bar\n" if eof() && !$f' file1 file2

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

##</code><code>##

--- cat before ---
line1
line2
line3
line1
line2
line3
--- perl output to stdout ---
line3
opt=bar
--- cat after ---
line1
line2
line3
line1
line2

##</code><code>##

--- cat before ---
line1
line2
line3
line1
line2
line3
--- perl output to stdout ---
--- cat after ---
line1
line2
line3
line1
line2
line3
opt=bar