use Win32::API;
use constant SPI_SETDESKWALLPAPER  => 20;
use constant SPIF_UPDATEANDSENDINI => 3;
my $image = "c:\\images\\image.bmp";

my $syspinf = Win32::API->new('user32','SystemParametersInfo',
  [ qw(I I P I) ], 'I')
  or die "Could not import function.\n";

$syspinf->Call(SPI_SETDESKWALLPAPER, 0, $image, SPIF_UPDATEANDSENDINI);