use Image::Magick;

my $img = Image::Magick->new;
$img->Read('./img.jpg' );
$img->Quantize( colors => 18 );
$img->Set( type => 'Palette' );
$img->Write('./out.png');

# I can check that it has been reduced... 

my $out = Image::Magick->new;
$out->Read('./out.png');
$out->Get( colors ) == 18 or die;