use Fcntl;
use POSIX qw(:unistd_h);
...

# $fh -- open file handle
# $lock is the structure that will be passed to fcntl().
# Note that values in the structure must be valid even 
# though they are not required for F_GETLK. If you just
# zero-out everything fcntl() will return "Invalid argument"
my $lock = pack 'sslli', F_UNLCK, SEEK_SET, 0, 0, 0;

# Second parameter - 5, represents F_GETLK. I couldn't use
# F_GETLK directly because it is mapped to 8 (F_GETLK64) and
# I couldn't get the definition for flock64 (the equivalent
# of "flock" for large files). The reason F_GETLK64 is 
# mapped to F_GETLK is because perl was compiled with
# USE_LARGE_FILES.
if (fcntl($fh, 5, $lock)){
   my $lpid = (unpack 'sslli', $lock)[-1];
   if ($lpid){
      print "File is locked by PID: $lpid\n";
   }else{
      print "File is not locked\n";
   }
}else{
   print STDERR "fcntl() failed: $!\n";
}

...