sub sys_seek {
   sysopen my $fh, "file.txt", O_RDONLY or die $!; 
   flock $fh, LOCK_EX or die $!; 
   print "Position: ", sysseek($fh, 0, SEEK_CUR), "\n";
   <$fh>;
   print "Position: ", sysseek($fh, 0, SEEK_CUR), "\n";
}

foo();