Perl RRD::Simple без отображения данных

#!/usr/bin/perl
use strict;
use warnings;

use RRD::Simple ();
use Data::Dumper;

$| = 1;    # Flush the output

my ($rrd, $unixtime, $file);

$file = "perl.txt";
my $path    = '/home/os/Desktop/Test_Perl/';
my $period  = '3years';
my $rrdfile = 'myfile.rrd';

while (sleep 15) {

  open(FH, ">>", $file) || die "Unable to open $file: $!\n";

  my $range   = 50;
  my $minimum = 100;

  my $random_number_in  = int(rand($range)) + $minimum;
  my $random_number_out = int(rand($range)) + $minimum;
  my $random_number_sec = int(rand($range)) + $minimum;

  # Create an interface object
  $rrd = RRD::Simple->new(
    file => $rrdfile,
    cf   => [qw( AVERAGE MIN MAX LAST )],
    #default_dstype => "DERIVE",
  );

  unless (-e $rrdfile) {

    # Create a new RRD file with 3 data sources called
    # bytesIn, bytesOut and faultsPerSec.
    $rrd->create(
      $period,
      step         => 5,         # 5 sec interval
      bytesIn      => "GAUGE",
      bytesOut     => "GAUGE",
      faultsPerSec => "GAUGE"
    );
  }

  # Put some arbitary data values in the RRD file for the same
  # 3 data sources called bytesIn, bytesOut and faultsPerSec.
  $rrd->update(
    bytesIn      => $random_number_in,
    bytesOut     => $random_number_out,
    faultsPerSec => $random_number_sec
  );
  print FH "This is the bytes_in: $random_number_in\n";
  print FH "This is the bytes_out: $random_number_out\n";
  print FH "This is the bytes_sec: $random_number_sec\n";

  # Generate graphs:
  #  /home/os/Desktop/Test_Perl/myfile-hourly.png, /home/os/Desktop/Test_Perl/myfile-daily.png
  #  /home/os/Desktop/Test_Perl/myfile-weekly.png, /home/os/Desktop/Test_Perl/myfile-monthly.png

  my %rtn = $rrd->graph(
    $rrdfile,
    destination => $path,
    basename    => "my_graph",
    timestamp   => "both",                       # graph, rrd, both or none
    periods     => [qw(hour day week month)],    # omit to generate all graphs
    sources          => [qw(bytesIn bytesOut faultsPerSec)],
    source_colors    => [qw(ff0000 aa3333 000000)],
    source_labels    => [("Bytes In", "Bytes Out", "Faults Per Second")],
    source_drawtypes => [qw(LINE1 AREA LINE)],
    line_thickness   => 2,
    extended_legend  => 1,
    title            => "Network Interface eth0",
    vertical_label   => "Bytes/Faults",
    width            => 800,
    height           => 500,
    interlaced       => "", # If images are interlaced they become visible to browsers more quickly
  );

  printf("Created %s\n", join(", ", map { $rtn{$_}->[0] } keys %rtn));

  # Return information about an RRD file
  my $info = $rrd->info($rrdfile); # This method will return a complex data structure containing details about the RRD file, including RRA and data source information.
  print Data::Dumper::Dumper($info);

  my @sources = $rrd->sources($rrdfile);
  my $seconds = $rrd->retention_period($rrdfile); # This method will return the maximum period of time (in seconds) that the RRD file will store data for.

  # Get unixtime of when RRD file was last updated
  $unixtime = $rrd->last($rrdfile);
  print FH "myfile.rrd was last updated at " . scalar(localtime($unixtime)) . "\n";

  # Get list of data source names from an RRD file
  my @dsnames = $rrd->sources;
  print "Available data sources: " . join(", ", @dsnames) . "\n";
  my $heartbeat_In  = $rrd->heartbeat($rrdfile, "bytesIn");
  my $heartbeat_Out = $rrd->heartbeat($rrdfile, "bytesOut");
  my $heartbeat_sec = $rrd->heartbeat($rrdfile, "faultsPerSec");  # This method will return the current heartbeat of a data source.

  printf "This is the heartbeat_in: %s\n", $heartbeat_In;
  my @rtn_In  = $rrd->heartbeat($rrdfile, "bytesIn",      10);
  my @rtn_Out = $rrd->heartbeat($rrdfile, "bytesOut",     10);
  my @rtn_sec = $rrd->heartbeat($rrdfile, "faultsPerSec", 10);    # This method will set a new heartbeat of a data source.

  close(FH);
}

Часть вывода:

'myfilerrd' => {
  'last_ds' => 'U',
  'value' => undef,
  'min' => '0',
  'max' => undef,
  'minimal_heartbeat' => 120,
  'index' => 3,
  'type' => 'DERIVE',
  'unknown_sec' => 15
}

Я не понимаю, почему значение не определено?