Please find the (simplified) program below. The file is simulated with an array. It is running itself.

#!/usr/bin/perl 
use strict;
use warnings;
$\="\n"; $,=", ";

#------------------------------------------------
my($f, $fpos, $fposlen, @data, $byte1, @byte2, $crc, @crcval); 
initCRC();
ReadDatFile();
exit;

#------------------------------------------------
sub ReadDatFile
{ # simulate file read:
  # three datasets:  0,1,2,3,4,5,6,7,8,9  and  10,11,2,3,4,5,6,7,8,9 a
+nd 20,11,2,3,4,5,6,7,18,19
  my @file=(0x00, 0xc0,0xff,0x80,1,2,3,4,5,6,7,8,9,0x8e,   10,0x80,0x8
+0,11,0x9d,   20,0xc0,0x01,0x80,18,19,0x1c,   0xff);
  $f.=chr($_)|'' foreach (@file);
  
  $crc=255; $fposlen=length($f); $fpos=0;
  @byte2=(0,0,0,0,0,0,0,0);
  readByte(); $data[0]=$_;                              # first data
  
  do
  { readByte(); $byte1=$_;                              # BYTE1: $_
    if ($byte1 & 0x80) {readByte(); $byte2[0]=$_; } 
    if ($byte1 & 0x40) {readByte(); $byte2[1]=$_; }
    if ($byte1 & 0x20) {readByte(); $byte2[2]=$_; }
    if ($byte1 & 0x10) {readByte(); $byte2[3]=$_; } 
    if ($byte1 & 0x08) {readByte(); $byte2[4]=$_; }
    if ($byte1 & 0x04) {readByte(); $byte2[5]=$_; }
    if ($byte1 & 0x02) {readByte(); $byte2[6]=$_; }
    if ($byte1 & 0x01) {readByte(); $byte2[7]=$_; }

    do_byte2( 0) if ($byte1 & 0x80); 
    do_byte2( 8) if ($byte1 & 0x40); 
    do_byte2(16) if ($byte1 & 0x20);
    do_byte2(24) if ($byte1 & 0x10); 
    do_byte2(32) if ($byte1 & 0x08); 
    do_byte2(40) if ($byte1 & 0x04); 
    do_byte2(48) if ($byte1 & 0x02); 
    do_byte2(56) if ($byte1 & 0x01); 
    
    readByte(); if ($crc>0) {print "ERROR: CRC (POS:$fpos)"; exit; }  
+ # crc check
    $crc=255;     

    print @data;           # ---- USE DATA ---------
    
    readByte(); $data[0]=$_;                         # next dataset
  } while ($_<255);
  
}

sub do_byte2
{ my $bpos=$_[0];                                     # pos in data fr
+om byte0
  my $pp=$byte2[$bpos>>3];
  if ($pp & 0x80) {readByte(); $data[$bpos+1]=$_;}    # take value int
+o data array
  if ($pp & 0x40) {readByte(); $data[$bpos+2]=$_;}
  if ($pp & 0x20) {readByte(); $data[$bpos+3]=$_;}
  if ($pp & 0x10) {readByte(); $data[$bpos+4]=$_;}
  if ($pp & 0x08) {readByte(); $data[$bpos+5]=$_;}
  if ($pp & 0x04) {readByte(); $data[$bpos+6]=$_;}
  if ($pp & 0x02) {readByte(); $data[$bpos+7]=$_;}
  if ($pp & 0x01) {readByte(); $data[$bpos+8]=$_;}
}

sub readByte
{ if ($fposlen==$fpos) {print "Last data structure not complete !"; ex
+it;}
  $_=vec($f,$fpos,8); 
  $crc=$crcval[$crc^$_];         # CRC     
  $fpos++;  
}

sub initCRC
{ for my $v (0..255)
  { $_=$v;
    for my $r (1..8)
    { if (($_&1)>0) {$_=($_>>1)^0x8c;}
      else          {$_=$_>>1;}  
    }
    $crcval[$v]=$_;  
} }
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Have you looked at Digest::CRC? It has the CRC routine itself written in C, so if you can supply the data to it in large chunks (instead of reading byte by byte), it should be much faster than Perl alone.