A quick and dirty way would be the following.. It uses the ImageMagick 'convert' command, although the Perl interface is much cleaner to use from inside Perl:

my $new_size = '640x480';
my $image_file = 'something.jpg';

my $new_image = `convert -size $new_size -resize $new_size $image_file
+ jpg:-`;

print "Content-type: image/jpeg\n\n$new_image";
[download]

The 'convert' line of code shows how to resize an image on the fly and print the new image to standard output (probably safest if you're doing CGI). Look through `man convert` for more explanation.

I would highly recommend using the Image::Magick interface, especially in a CGI application. If you're just doing some ad-hoc housekeeping scripts, you may not even need Perl, just some bash scripting.