This is surprising, as HTTP::Server::Simple should then die in the parent with Can't fork: ..., which you should have seen:

sub background {
    my $self  = shift;
    my $child = fork;
    croak "Can't fork: $!" unless defined($child);
    return $child if $child;

    srand(); # after a fork, we need to reset the random seed
             # or we'll get the same numbers in both branches
    if ( $^O !~ /MSWin32/ ) {
        require POSIX;
        POSIX::setsid()
            or croak "Can't start a new session: $!";
    }
    $self->run(@_); # should never return
    exit;           # just to be sure
}
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So, either you're looking at a webserver that is not the webserver you launched, or the $pid is not where you store the information.

If you're trying to kill the child from within the child, $pid will also not be defined there (see fork). To kill yourself, use $$ as the pid.

Another possibility is that the op is trying to launch the webserver in one process invocation, and then kill it in another. In which case $pid will have to be written to/from a file.