Lookahead alone won't do because the pair might be cut into two reads. It does make things more complicated.

I don't know anything about surrogates. I assumed the following: 

#!/usr/bin/perl

# usage:
#    fix_surrogates.pl < infile > outfile

# Hi Surrogate: D800-DBFF
# Lo Surrogate: DC00-DFFF

use strict;
use warnings;

binmode STDIN;   # Disable :crlf
binmode STDOUT;  # Disable :crlf

my $read_size = 16*1024;

my $valid_pat   = qr/ .[^\xD8-\xDF]
                    | .[\xD8-\xDB].[\xDC-\xDF]
                    /xs;

my $invalid_pat = qr/ .[\xDC-\xDF]
                    | .[\xD8-\xDB](?=.[^\xDC-\xDF])
                    /xs;

my $ibuf = '';
my $obuf = '';

for (;;) {
   my $rv = read(STDIN, $ibuf, $read_size, length($ibuf));
   die("$!\n") if !defined($rv);
   last if !$rv;

   for ($ibuf) {
      /\G ($valid_pat+) /xgc && do { $obuf .= $1;              };
      /\G $invalid_pat  /xgc && do { $obuf .= "\xFD\xFF"; redo };
   }

   print($obuf);

   $ibuf = substr($ibuf, pos($ibuf)||0);
   $obuf = '';
}

$ibuf =~ s/..?/\xFD\xFF/sg;   
print($ibuf);

[download]

Update: Tested. Fixed character class that wasn't negated as it should have been. 

>type testdata.pl
binmode STDOUT;
my $hi = "\xF4\xDB";
my $lo = "\xE2\xDE";
print "a\0" . $hi . $lo   . "b\0" . "\n\0",
      "c\0" . $hi . "c\0" . "d\0" . "\n\0",
      "e\0" . $lo . "f\0" . "g\0" . "\n\0";

>perl testdata.pl | perl fix_surrogates.pl | perl -0777 -pe"BEGIN { bi
+nmode STDIN, ':encoding(UTF-16le)'; binmode STDOUT, ':encoding(US-ASC
+II)' }"
"\x{10d2e2}" does not map to ascii, <> chunk 1.
"\x{fffd}" does not map to ascii, <> chunk 1.
"\x{fffd}" does not map to ascii, <> chunk 1.
a\x{10d2e2}b
c\x{fffd}cd
e\x{fffd}fg

[download]

In reply to Re^3: Handling malformed UTF-16 data with PerlIO layer by ikegami
in thread Handling malformed UTF-16 data with PerlIO layer by almut

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