imager rounded corner mask with transparency

rounded.pl

sub rounded_mask {
    my ($img, $rad) = @_;
    state $ss = 4; # subsample smooth
    my $srad = int $rad*$ss;
    my $corner = Imager->new(xsize => $srad, ysize => $srad, channels => 1)
        ->arc(qw'x 0 y 0 d1 0 d2 90', r => $srad - $ss/2)
        ->scale(scalefactor => 1/$ss);
    my ($w, $h) = ($img->getwidth, $img->getheight);
    my $mask = Imager->new(xsize => $w, ysize => $h, channels => 1);
    $mask->box(box => [$rad, 0, $w - 1 - $rad, $h - 1], filled => 1);
    $mask->box(box => [0, $rad, $w - 1, $h - 1 - $rad], filled => 1);
    $mask->paste(left => $w - $rad, top => $h - $rad, img => $corner); # br
    $mask->paste(top => $h - $rad, img => $corner->flip(qw'dir h')); # bl
    $mask->paste(img => $corner->flip(qw'dir v')); # tl
    $mask->paste(left => $w - $rad, img => $corner->flip(qw'dir h')); # tr
    my $out = Imager->new(xsize => $w, ysize => $h, channels => 4);
    $out->compose(src => $img, mask => $mask);
}