$canvas{width} = 8.5;
$canvas{height} = 11;
foreach(@pictures_to_canvas){
    if ( ($canvas{width} > $$_{width}) && ($canvas{height} > $$_{width}) {
        $canvas{width} -= $$_{width};
        $canvas{height} -= $$_{height};
    }
#Throw exception here if height or width goes below a min value.
}
...