#!/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