Your answer is not only at least a little bit right, it's exactly on target!  Excellent analysis! ++

I read your response yesterday, and was fairly sure that you had hit the nail on the head.  But I wanted to do it justice by testing further, and as I expected, the program succeeds when another reference is created to the filehandle.

First, I made sure $b_use_global_fh was zero, and changed system_command() to return both $psyscmd and the filehandle $fh, so I could create a reference to $fh to keep it open: 

    my ($psyscmd, $fh) = system_command("$bld_cmd 2>&1");
    $global_fh = $fh;

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That worked exactly as you predicted.  It also worked when I simply had the parent process return the filehandle: 
    if ($pid) {
        # Parent
        print STDERR "\e[101m(1a) Parent about to return\e[m\n";
        return $fh;
    }

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but only, of course, when the reference was preserved after the return from monitor_build: 
my $unused_fh = monitor_build("./fakebuild");

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So I thank you for a very enlightening solution.  It taught me something important about closures; namely, that they close their open filehandles which go out of scope, just the way a program does when it exits.  Nicely written!

Update:  I just realized (and just tested as correct) that another way to keep the filehandle from attempting to close is for the parent to return the reference to the closure: 

    my $psyscmd = system_command("$bld_cmd 2>&1");
    
    # ...

    if ($pid) {
        # Parent
        print STDERR "\e[101m(1a) Parent about to return\e[m\n";
        return $psyscmd;
    }

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Then, as long as it is saved in the main program, this technique works as well.  It also avoids having to fuss with the filehandle directly.
@ARGV=split//,"/:L"; map{print substr crypt($_,ord pop),2,3}qw"PerlyouC READPIPE provides"

In reply to Re^2: Why is a deeply-bound lexical in a forked closure blocking the parent process? by liverpole
in thread Why is a deeply-bound lexical in a forked closure blocking the parent process? by liverpole

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