If I understand the Image::Grab docs correctly, it just returns the image as binary data. According to the docs, Image::Magick supports reading filehandles, which you could use with an in-memory filehandle (open), as well as reading data in memory directly:

my $image = Image::Magick->new(magick=>'jpg');

open my $fh, '<', \$imgdata or die $!;
$image->Read(file=>$fh);
# - or -
$image->BlobToImage($imgdata);
[download]

(untested)