use File::Find;
@ARGV = ();
find sub {
  push @ARGV, $File::Find::name if -f and /\.html$/;
}, ".";
{
  local $^I = ".bak";
  local $/;
  while (<>) {
    if (s/foo/bar/g) { # changes?
      print; # print the new one
    } else { # no changes?  back it out!
      close ARGVOUT; # for windows, not needed on Unix
      rename "$ARGV$^I", $ARGV or warn "Cannot rename for $ARGV$^I: $!";
    }
  }
}