What would you do if you had an occation where the data wasn't on cut (ie. the first pattern was offset by 10 bytes) and the user had no way of knowing?

This reads a two framesized chunk and uses a regex to discover the alignment of the first full frame within it. If the offset is non-zero, then it discards that many bytes from the front of buffer (and issues a warning to indicate that), tops up the buffer to two full frames thereby aligning the read pointer to the start of the 3rd frame. Processes and outputs the first two full frames and then processes the rest a frame at a time as before.

#! perl -sw
use strict;
use bytes; 

open IN,  '< :raw', $ARGV[ 0 ] or die "$ARGV[ 0 ] : $!";
open OUT, '> :raw', $ARGV[ 1 ] or die "$ARGV[ 1 ] : $!";

## Grab a double buffer load first time so we can check & correct alig
+nment
local $/ = \768; 

my $buf = <IN>; ## Read two frames worth

## Check alignment. Assumes the xf4 .191 xf4 is unique per frame?
$buf =~ m[(\xF4.{191}\xF4)];

## Record the offset to the first frame
my $offset = $-[0];

## If there was an offset to the first match
if( $offset != 0 ) {
    ## Chop off the leading junk
    substr( $buf, 0, $offset, '' );
    
    ## Top up the buffer to two full frames
    read( IN, $buf, $offset, 768 - $offset );
    
    warn "$offset bytes discarded from front of file.";
}

## Process the first two whole frames
print OUT unpack 'x2 a190 x2 a58 x132' x 2, $buf

## Now process as before
local $/ = \384; ## Read file in 384 byte chunks.


while( <IN> ) {
    print OUT unpack 'x2 a190 x2 a58', $_;
}

close IN;
close OUT;
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