use strict;
use warnings;
use Fcntl qw/ SEEK_CUR O_RDONLY LOCK_EX /;

sub sys_verwenden {
   print '-' x 30, "\n";

   sysopen my $fh, "file.txt", O_RDONLY or die $!; 
   flock $fh, LOCK_EX or die $!; 

   sysread($fh, my $buf, 50);
   my $pos = sysseek($fh, 0, SEEK_CUR);
   print "Position: $pos\n";
   print "Buffer:\n";
   print $buf, "\n";
   close $fh;
}

sub vermischt {
   print '-' x 30, "\n";

   sysopen my $fh, "file.txt", O_RDONLY or die $!; 
   flock $fh, LOCK_EX or die $!; 

   while (my $line = <$fh>) {
      printf "%5s %s", sysseek($fh, 0, SEEK_CUR), $line;
   }   

   close $fh;
}

sub ohne_sys {
   print '-' x 30, "\n";

   sysopen my $fh, "file.txt", O_RDONLY or die $!; 
   flock $fh, LOCK_EX or die $!; 

   while (my $line = <$fh>) {
      printf "%5s %s", tell($fh), $line;
   }   

   close $fh;
}

sys_verwenden();
vermischt();
ohne_sys();