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sub daemonize {
    # try to daemonize
...
    return errstr
    }
}

$isdaemon = daemonize()
if ($isdaemon ne "1") {
    print "error: $isdaemon";    
}

if (! daemonize) {
    report error
}

open($fh, file) or die "$!";

use Scalar::Util;
sub daemonize {
...
    return dualvar 0, errstr
    }
}

$isdaemon = daemonize;
if ($isdaemon == 0) {
    print "error: $isdaemon";
}