#!perl -i

# or:
# $^I = '';

@ARGV = ('myfile');

while (<>) {
  s/foo/bar/;
  print;
}

##</code><code>##

perl -pi -e 's/bar/foo/g' myfile

##</code><code>##

if (/foo/) {
  s/foo/bar/g;
}

s/foo/bar/g;