First of all, I don't see the need for use CGI ":all"; for it seems a lot of wasted resources to me ;) If you only going to print the header, I'd suggest not to use CGI.pm at all (print "Content-Type: image/png\n\n";)

Secondly, besides this code, you could also use: Image::Thumbnail, Image::Magick::Thumbnail, Image::GD::Thumbnail, or even Apache::GD::Thumbnail ... CPAN is your friend here ;)