You have to know two things:

1. the mime type of the image
2. the size of the image

Then it's all a matter of sending the correct HTTP header, and then simply printing the image.

use CGI qw(:standard);

my $mime_type = 'image/jpeg';
my $size = length($data); # Assuming $data contains your image

print header(
    -type => $mime_type,
    -Content_Length => $size,
);
print $data;
[download]

Now, how you get the mime type and the image is up to you, it could be from a database or from a file on disk.