# Two useful auxilliary hashes
my %trans_in = (
  "\xFD" => "\xFD",
  "\xFE" => "\xFD\xFE",
  "\xFF" => "\xFD\xFE\xFE",
);
my %trans_out = reverse %trans_in;

# On the way in...
$string =~ s/(\xFD|\xFE|\xFF)/$trans_in{$1}/g;

# On the way out
$string =~ s/(\xFD\xFE*)/$trans_out{$1}/g;