I am not sure if you mean use Perl to pull stuff from other servers:

use LWP::Simple 'getstore';
my $url = 'http://some.domain.com/image.gif';
my $file = '/tmp/image.gif';
getstore( $url, $file );
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or if you mean do the opposite of a file upload, ie use a CGI script to control the downloading of certain images. If you don't want control you would just make them browsable. Anyway if you do a Super Search for 'CGI file download filename' in the text you will turn up all the info you need including:

Web Initiated File Download
File Download
CGI File Download

Incidentally when you open a file using >> you are opening it in APPEND mode, ie if it exists you add to the end of it. You almost certainly want > Also check the return value of the open and apply binmode as in:

open F, ">$file" or die "Could not write $file Perl says reason $!\n";
binmode F;   # may be important does no harm it not needed
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