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sysopen my $fh,$file,O_WRONLY|O_CREAT|O_NONBLOCK|O_NOCTTY
  or croak("Can't create $file : $!");

my $SYSOPEN_MODE = O_WRONLY|O_CREAT;
if(exists $Fcntl::{'O_NONBLOCK'}){
...
    $SYSOPEN_MODE |= &{ $Fcntl::{'O_NOCTTY'} };
}