use HTML::Template::Compiled;
sub stream {
    my ($fh) = @_;
    while (<$fh>) {
        print "STREAM: $_";
        sleep 1;
    }
}

if (my $pid = open my $pipe, "-|") {
    # prozess forken, kind schreibt in $pipe als STDOUT,
    # wir lesen von $pipe
    stream($pipe);
}
else {
    my $htc = HTML::Template::Compiled->new(
        scalarref => \<<EOM,
test
1
zwo
blubb
EOM
        out_fh => 1,
    );
    $htc->output(\*STDOUT); # schreibt in $pipe
    exit;
}

# Seminartermin für Outlook-Kalender ausgeben
sub SendeOutlookTermin {
        my $self = shift;

        $self->{ICALENDAR_TMPL} = HTML::Template::Compiled->new (
                filename => 'htc_icalendar.ics',
                path     => '/usr/local/httpd/htdocs/cgi-bin/seminare_v2/tmpl/',
                );

        $self->ICalendar::HoleIcalTerminDaten();

        $self->{DOWNLOAD} = 'files/'.$self->session->id().'.ics';

        open (ICAL, ">$self->{DOWNLOAD}") || die 'Fehler bei open \n';
                print ICAL $self->{ICALENDAR_TMPL}->output();
        close(ICAL) || die 'Fehler bei close \n';

        $self->{DateiDownloadTyp} = 'ics';

}


# Seminartermin für Outlook-Kalender aus DB auslesen
sub HoleIcalTerminDaten {
        my $self = shift;

        $self->SqlStatements::HoleIcalTerminDatenSQL() unless defined $self->{HoleIcalTerminDatenSQL};
        $self->{HoleIcalTerminDatenSQL}->execute( $self->{TerminID} );

        my ( $seminar_nr, $seminar_titel, $start_datum, $start_uhrzeit, $ende_datum, $ende_uhrzeit, $start_datum_ical, $start_uhrzeit_ical, $ende_datum_ical, $ende_uhrzeit_ical );
        $self->{HoleIcalTerminDatenSQL}->bind_columns(undef, \( $seminar_nr, $seminar_titel, $start_datum, $start_uhrzeit, $ende_datum, $ende_uhrzeit, $start_datum_ical, $start_uhrzeit_ical, $ende_datum_ical, $ende_uhrzeit_ical ));
        $self->{HoleIcalTerminDatenSQL}->fetch();

        $self->{ICALENDAR_TMPL}->param(
                CGISESSID => $self->session->id(),
                SEMINAR_NR => "$seminar_nr",
                SEMINAR_TITEL => "$seminar_titel",
                SEMINAR_VON => "$start_datum $start_uhrzeit",
                SEMINAR_BIS => "$ende_datum $ende_uhrzeit",
                SEMINAR_VON_DATETIME => "$start_datum_ical"."T$start_uhrzeit_ical",
                SEMINAR_BIS_DATETIME => "$ende_datum_ical"."T$ende_uhrzeit_ical",
                JETZT_DATETIME => "20081006T080000",
                );
}

...
use CGI::Application::Plugin::Stream (qw/stream_file/);
...
$self->stream_file($self->{DOWNLOAD}, 2048) || die 'Fehler bei stream_file \n';
$self->header_add( -attachment => "kalender.ics" );

use IO::String;

#...

$self->{DOWNLOAD} =IO::String->new($self->{ICALENDAR_TMPL}->output());

stat() on unopened filehandle GEN0 at /usr/lib/perl5/site_perl/5.8.6/CGI/Application/Plugin/Stream.pm line 21., referer: https://xxx/seminare_v2/kalender
Failed to load File::MMagic module to determine mime type: Can't locate object method "BINMODE" via package "FileHandle" at /usr/lib/perl5/site_perl/5.8.6/File/MMagic.pm line 576., referer: https://xxx/seminare_v2/kalender
Error executing class callback in postrun stage: Can't locate object method "SEEK" via package "FileHandle" at /usr/lib/perl5/site_perl/5.8.6/CGI/Application/Plugin/Stream.pm line 84., referer: https://xxx/seminare_v2/kalender
malformed header from script. Bad header=Content-length: seminare.pl, referer: https://xxx/seminare_v2/kalender