After reading through the various previous replies and the linked material I put together the following two test applications. The CGI app is heavily based on the sample code in merlyn's LinuxMag column Watching long processes through CGI (pointed to by McDarren++), with amendments so that it works under Windows (but no longer under *nix, although making both work in this "framework" ought be trivial). In the real application the CGI app would manage the monitored app by writing to a second file.

The managing app:

#!Perl -w
use strict;
use CGI::Pretty qw(:standard :cgi-lib);
use CGI::Carp qw(fatalsToBrowser); # Remove for production code
use File::Cache;

$CGI::DISABLE_UPLOADS = 1;         # Disable uploads
$CGI::POST_MAX        = 10240; # Maximum number of bytes per post

$| = 1; # Unbuffered output

if (param('Spawn')) {
    # setup monitoring page then spawn the monitored process
    my $session = get_session_id();
    my $cache = get_cache_handle();
    
    $cache->set($session, "wait ..."); # no data yet
    Delete_all();
    
    # parent redirects browser to monitor session
    param('session', $session);
    print redirect (self_url());
    close STDOUT;

    # Rest of this block alters in *nix context to spawn monitored pro
+cess
    use Win32::Process;
    
    my $job;
    
    Win32::Process::Create (
        $job,
        'c:/Perl/bin/perl.exe', "perl.exe spawned.pl $session",
        0,
        NORMAL_PRIORITY_CLASS | DETACHED_PROCESS,
        '.'
        );

    exit 0; # all done
  
} elsif (my $session = param('session')) {
    # display monitored data
    my $cache = get_cache_handle();
    my $data = $cache->get($session);
    
    if (! $data) { # something is wrong
        showError ("Cache data not available");
        exit 0;
    }
    
    my $headStr = $data eq 'Completed' ? '' : "<meta http-equiv=refres
+h content=5>";
    print header();
    print start_html (-title => "Spawn Results", -head => [$headStr]);
    print h1("Spawn Results");
    print pre(escapeHTML($data));
    print end_html;
  
} else {
    # display spawn form
    print header(), start_html("Spawn"), h1("Spawn");
    print start_form();
    print submit('Spawn', 'spawn');
    
    my %params = Vars ();
    for my $param (keys %params) {
        print br ("$param -> $params{$param}");
    }
    
    print end_form(), end_html();
}

exit 0;


sub showError {
    print header(), start_html("SpawnError"), h1("Spawn Error");
    print p (shift);
    
    my %params = Vars ();
    for my $param (keys %params) {
        print br ("$param -> $params{$param}");
    }
    
    print end_html();
}


sub get_cache_handle {
    
    File::Cache->new
        ({
          namespace => 'Spawn',
          username => 'nobody',
          default_expires_in => '30 minutes',
          auto_purge_interval => '4 hours',
         });
}


sub get_session_id {
    require Digest::MD5;

    Digest::MD5::md5_hex(Digest::MD5::md5_hex(time().{}.rand().$$));
}
[download]

The monitored app:

#!Perl -w
use strict;
use File::Cache;

my $session = shift;
my $cache = get_cache_handle();

$cache->set($session, "wibble ..."); # no data yet

my $end = time () + 20;
my $count = 0;

while (time () < $end) {
    $cache->set ($session, "Count: $count\n");
    ++$count;
    sleep (1);
}

$cache->set ($session, "Completed");
exit 0; # all done

sub get_cache_handle {
    
    File::Cache->new
        ({
          namespace => 'Spawn',
          username => 'nobody',
          default_expires_in => '30 minutes',
          auto_purge_interval => '4 hours',
         });
}
[download]

The code was tested using a local Apache server and seems to provide exactly the types of interaction I am looking for.

Are there any glaring oversights?