i think this will work for you:

/\x26\x67(?!\x26\x67)(.(?!\x26\x67)){6}.{2}/s
[download]

Based upon this test code to try to match ab followed by 4 bytes w/no ab sequence:

 perl -le 'print for map { /ab(?!ab)(.(?!ab)){2}.{2}/s ? $& : () } @AR
+GV' abhjabdf abasdasd abdasdadsab ababblah
OUTPUTS:
abasda
abdasd
abblah
[download]

Update: Here's a commented version:

  /
    \x26\x67     # Starting sequence (let's call it AB)
    (?!\x26\x67) # Exclude double starting seqeuence, e.g. ABAB
    (?:          # non-capture
      .(?!\x26\x67) # Any character that's not followed by the sequenc
+e
    ){6}         # Need N-2 of these
    .{2}         # And the last two characters can be anything.
                 # note we know they're not AB because otherwise the
                 # (N-2)th item in the previous part wouldn't have mat
+ched
  /sx            # Enable comments, and make . mean everything
[download]

Update: Here's a generic version:

  my $marker = '&g';
  my $N = 8;
  my $n1 = $N-2;
  /$marker(?!$marker)(?:.(?!$marker)){$n1}.{2}/s
[download]

Update: (As pointed out by diotalevi) Added the /s modifier in case the data stream can have a newline in it.