use Image::Magick;

my $image = new Image::Magick;

$image->Read('foo.png');

my $count;                                     # The number of non-transparent pixels.
my $transparent = $image->Get('transparent');  # Index of the transparent colour.
my $height      = $image->Get('height');       # Image height.
my $width       = $image->Get('width');        # Image width.

for my $x ( 1 .. $height ) {
    for my $y ( 1 .. $width ) {
        $count++ if $image->Get("pixel[$x,$y]") != $transparent;
    }
}