I'm facing the same problem, though not in a testing situation.

Following Corion's advice, I'm trying to pass the server's PID (returned by its background() method) to a handler that will be responsible for killing it.

My problem is that I can't find a way to pass the PID to the handler.

I've tried several things which all end up the same way when I access the url associated to the handler: the PID's value has not been passed and Perl "Can't kill a non-numeric process ID".

I've tried this (where the setup is maximally contrived for brevity):

A) Store PID as a global in the main package and access it from the server package.

use strict;
use warnings;

use MyWebServer;

my  $server = MyWebServer->new(8080);
our $pid    = $server->background();
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package MyWebServer;

use strict;
use warnings;

use HTTP::Server::Simple::CGI;
use base qw(HTTP::Server::Simple::CGI);

sub handle_request {
    kill 9, $main::pid;
}

1;
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B) Store PID as a global in the server package and set it in the main package.

use strict;
use warnings;

use MyWebServer;

my $server        = MyWebServer->new(8080);
$MyWebServer::pid = $server->background();
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package MyWebServer;

use strict;
use warnings;

our $pid;

use HTTP::Server::Simple::CGI;
use base qw(HTTP::Server::Simple::CGI);

sub handle_request {
    kill 9, $pid;
}

1;
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C) Store the PID in a separate module used by test.pl and webserver.pm.

use strict;
use warnings;

use Pid;
use MyWebServer;

my $server = MyWebServer->new(8080);
Pid->set($server->background());
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package MyWebServer;

use strict;
use warnings;

use Pid;

use HTTP::Server::Simple::CGI;
use base qw(HTTP::Server::Simple::CGI);

sub handle_request {
    kill 9, Pid->get();
}

1;
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package Pid;

use strict;
use warnings;

my $pid;

sub get {
    my $package = shift;
     return $pid;
}

sub set {
    my $package = shift;
    $pid = shift;
}

1;
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I've tried even more esoteric things until I started to feel like Wile E. Coyote, so I think it's time I submit this problem to you.

Any advice would be welcome, and so would any explanation as to what I fail to understand.