lock_screen_background.pl

#!/usr/bin/env perl 

use strict;
use warnings;
use utf8;
use Encode;
use Net::DBus;
use diagnostics;

sub dbus_sub
{
    # Reusable subroutine for calling
    # dbus methods

    my $bus = Net::DBus->session;
    my $service = $bus->get_service($_[0]);
    my $object = $service->get_object($_[1],$_[2]);
    my $method = $_[3];
    my $method_out = $object->$method;
    return $method_out;
}

sub is_locked
{
    # wrapper subroutine for 
    # testing whether the 
    # use session is locked

    my $val = dbus_sub( 'com.canonical.Unity', 
                        '/com/canonical/Unity/Session',
                        'com.canonical.Unity.Session',
                        'IsLocked'
                      );
    return $val;
}
my $prefix = "file://";
my $lock_background = $prefix .  $ARGV[0];
my $original = qx/gsettings get org.gnome.desktop.background picture-uri /;
my @set_new=('gsettings', 'set', 
             'org.gnome.desktop.background', 'picture-uri',
             $lock_background
            );
system(@set_new);
dbus_sub( 'com.canonical.Unity',
          '/com/canonical/Unity/Session',
          'com.canonical.Unity.Session',
          'Lock');

sleep(1);
while(  is_locked() )
{
    $| = 1;
    print  "waiting\n";
    sleep(1);
}

my @restore_original = ('gsettings', 'set', 
                        'org.gnome.desktop.background', 
                        'picture-uri',
                        $original
                       );
system(@restore_original);