I haven't done much with Image::Magick either, but this is trivial with GD...

use GD;

my $img = GD::Image->new( $width, $height );

# The first color allocated becomes the background color
$img->colorAllocate( 255, 0, 0 );

# $img->jpeg will return the image data, you can then
# save it to a file, or print it with an image/jpeg
# header or whatever you want to do...
print $img->jpeg();
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