#! /usr/bin/env perl
use strict;
use warnings;

use CGI;
use File::Basename qw( basename );


my $generated_file = "/tmp/x.png";

# here happens the magic and $generated_file is generated

my $cgi = CGI->new();

print $cgi->header(
    '-type'       => 'image/png',
    '-attachment' => basename($generated_file),
);


open my $fh, '<', $generated_file or die "open($generated_file) failed: $!";
binmode($fh);               # for binary data
{
    local $/ = \1024;       # deliver in 1 kiB chunks
    print $_ for <$fh>;     # deliver data
}
close $fh;

#! /usr/bin/env perl
use strict;
use warnings;

use CGI;

my $fake_filename = "x.png";

my $cgi = CGI->new();
print $cgi->header(
    '-type'       => 'image/png',
    '-attachment' => $fake_filename,
);

# let's assume, generate_png() generates a valid PNG data stream
print generate_png();