I'm not really clear on what you want to do. The basic usage of fork is as follows:

if (my $child = fork()) {
    # in parent
    while (! -f "output.txt") {
        print "Waiting for output.txt\n";
        sleep 10;
    };
    print "Found output.txt, continuing\n";
} else {
    # In child, just run p2
    exec 'p2';
    # exit
};

# do more stuff

print "Waiting for child to finish\n";
wait $child;
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