I still need to figure out how to selectively close those handles that correspond to STDERR and STDOUT,

Would it not be sufficient to simple close all open files in your child? 

use POSIX ();

...
}
else { ## child
    POSIX::close( $_ ) for 0 .. 255; ## Or higher if that's a possibil
+ity
'''
}

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In reply to Re^3: How to find all open STDERR and STDOUT dups? by BrowserUk
in thread How to find all open STDERR and STDOUT dups? by tlm

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