Folks-

Wow, no responses. Either this is a hard question, or nobody cares about doing something goofy like this :-)

After playing around with things, I've decided to go with POE::Filter::Block because it provides a way to override the default encode and decode routines with ones of your own. The following code illustrates how using POE::Filter::Block in standalone mode, works fine for extracting variable length binary messages beginning with 0xfefefefe (however using it as a filter on POE::Wheel::FollowTail still doesn't work - more on this later).

Update: Fixed buffering bug (removed '|$' in regular expression)

use strict;
use warnings;

my $data='ThisIsBeginningGarbagefefefefe037e01000001000a666c7832313100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';

$data = pack('H*', $data);

use POE::Filter::Block;

sub _myencode { print STDERR "_myencode\n" };
sub _mydecode {
  my $stuff = shift;
  print STDERR "M1: ", unpack('H72', $$stuff), "...\n";

  # Extract everything through 1st 0xfe{4} flag, up to the 2nd...
  $$stuff =~ /^(.*?\xfe{4}.*?)(\xfe{4})/s || return;

  # Return its measured length (in characters)...
  my $length = length($1);
  return($length);
}

my $filter = POE::Filter::Block->new( LengthCodec =>
    [ \&_myencode, \&_mydecode ]
);

$filter->get_one_start([ $data ]);

while (1) {
  my $block = $filter->get_one();
  last unless @$block;
  print STDERR "M2: ", unpack('H72', $block->[0]), "...\n",
    length($block->[0]), " characters\n";
}
[download]

My output looks like this (the first message includes the 'ThisIsBeginningGarbage' stuff by design) (truncated for eye appeal):

M1: d12c2cbe02772700abba0efefefefe037e01000001000a666c7832313100000000
M2: d12c2cbe02772700abba0efefefefe037e01000001000a666c7832313100000000
71 characters
M1: fefefefe037e01000001000a666c78323131000000000000000036bc0200000149
M2: fefefefe037e01000001000a666c78323131000000000000000036bc0200000149
440 characters
M1: fefefefe037e01000001000a666c78323131000000000000000036bc0200000149
M2: fefefefe037e01000001000a666c78323131000000000000000036bc0200000149
250 characters
M1: ...
[download]