#!/usr/bin/perl -i.bak -w
@ARGV = ('c:/test.txt');             # set our file to process
while (<>) {
   print "newline\n" if m/somecond/; # add a new line is line matches somecond
   s/this/that/;                     # substitute this for that on every line
   print;                            # write the modified (or not) lines
}