use Fcntl qw(:seek);

  open (FOO, "+<$foo") or die "Can't open $foo: $!";
  my $slurp = join '', <FOO>;
  $slurp =~ s/hello/goodbye/sgi;
  seek FOO, 0, SEEK_SET;
  print FOO $slurp;
  truncate FOO, tell FOO;
  close FOO;