if ($res==0) {print "OK\n"; last;};

use strict;
use warnings;

my $diafile="RT_4.dia";
open(FDIA, $diafile) or die "cannot open $diafile";

my $read_len = 8;
my $tot_len  = 0;
my $file_len = (stat($diafile))[7];
print "Dateigroesse $file_len Bytes\n";

while ( my $len = sysread(FDIA, my $rec, $read_len) ) { # bis eof lesen
   if (!defined $len) {
      next if $! =~ /^Interrupted/;
      die "system read error: $!";
   }   

   # $rec Verarbeitung

   $tot_len += $len;
   warn "read $tot_len/$file_len\n";
}