open(FH,"myfile") or die "e tu brutus?\n"; # clobber it.
while(<FH>)
  if ($_ =~ m/foo/){
    $line = $_;
    $line =~ s/foo/bar/g;
  }
  else{
    $line = $_;
  }
  $file = $file . $line;
}
open (FH,"> myfile") or die "e tu brutus?\n";
print FH $file