## Open our file for reading and writing:
open(MYFILE, "+< $myfile") or die "Could not open $myfile: $!\n";

## File locking would be a Good Idea here

## Loop through the file one line at a time
while(<MYFILE>) {

  if (/$foo/) { ## We have a match!
    my $tellme = tell(MYFILE)-length $_; ## See below

    my @baz = <MYFILE>; ## Slurp everything from that point forward

    unshift(@baz,$_); ## Throw our string back in

    ## Above, we stored the location in the file where the 
    ## first line was that matched. Now we rewind to that point.
    seek(MYFILE,0,$tellme);

    ## The file from where we are (via seek) to the end of 
    ## the file is now in memory, inside @baz
    for (@baz) {
      s/$foo/$bar/;
      print MYFILE $_;
    }

    ## Now we must cleanup our file, especially if the 
    ## file has shrunk from our changes:
    truncate (MYFILE,tell(MYFILE));

    last; ## Bail from the original MYFILE loop
  }
}