This works for me:

#!/usr/bin/perl -w

use strict;
use warnings;
use CGI::Carp qw{ fatalsToBrowser };
use IO::File;

my $png    = "test.png";
my $fh     = new IO::File($png) or die "Can't read file '$png' ($!)\n"
+;
my $nbytes = (-s $png);
binmode $fh;
my $data;
sysread($fh, $data, $nbytes) or die "Failed to read data from '$png' (
+$!)\n";

print "Content-disposition:  attachment; filename=$png\n\n";
print $data;
[download]

Update:  Removed '...' from .png filename in header.

You can suggest the browser to offer the thing for download with the Content-Disposition HTTP header (but browser can ignore that).

Update...
Never mind, that will just load the image.

That's really more of a browser question shading into HTTP/HTML, rather than a perl question. Somebody here might know, but searching web design sites would probably get you answers quicker.