#!/usr/bin/perl --
use strict;
print "Content-type: application/octet-stream\n";
print "Content-disposition: attachment; filename=big-test.csv\n\n";
print "Test,One,Two,Three\n";
exit;

##</code><code>##

my $app = sub {
    return
    [ ## response
        [ ## headers
            'Content-Disposition' => 'attachment; filename="$filename"',
            'Content-length' => '3',
            'Content-Type' => '$mimetype',
        ],
        [ ## content
          123
        ]
    ];
};

##</code><code>##

#!/usr/bin/perl --
use strict;
use warnings;
use CGI ( );
binmode STDOUT;
my $q = CGI->new;
print $q->header(
    -type => $mimetype,
    -attachment => $filename,
    -Content_length => -s $filehandle,
);
...

##</code><code>##

#!/usr/bin/perl --
use strict;
use warnings;
use CGI qw( param header );
binmode STDOUT;
print header(
    -type => $mimetype,
    -attachment => $filename,
    -Content_length => -s $filehandle,
);
...