it looks like the problem is $AUTOLOAD in Fcntl, which throws the error when the symbol table is queried.

looping through the symbol table doesn't throw the same error:

foreach my $sym (keys $Fnctl::){
  if($sym eq 'O_NONBLOCK'){
    $SYSOPEN_MODE |= &{ $Fcntl::{'O_NONBLOCK'} };
  }
}
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however, this doesn't work with another constant, O_NOCTTY