Then watch() is not a good fit for your requirements. All you need is something like:

my $job = Win32::Job->new;
my $pid = $job->spawn();

while( 1 ) {
    ## read and process next task

    sleep 3 while $job->status->{ $pid }{ exitcode } == $STILL_RUNNING
+ ## 259??
            and not -e 'killfile';

    $job->kill if $job->status->{ $pid }{ exitcode } == $STILL_RUNNING
+;

    $job = Win32::Job->new;
    $pid = $job->spawn( ... )
}
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