how do I un-OR OR results, against the other flags, to get back to these basic constants?

(From MSVC header files:

#define _O_RDONLY       0x0000  /* open for reading only */
#define _O_WRONLY       0x0001  /* open for writing only */
#define _O_RDWR         0x0002  /* open for reading and writing */
#define _O_APPEND       0x0008  /* writes done at eof */
#define _O_CREAT        0x0100  /* create and open file */
#define _O_TRUNC        0x0200  /* open and truncate */
#define _O_EXCL         0x0400  /* open only if file doesn't already e
+xist */
#define _O_TEXT         0x4000  /* file mode is text (translated) */
#define _O_BINARY       0x8000  /* file mode is binary (untranslated) 
+*/
#define _O_WTEXT        0x10000 /* file mode is UTF16 (translated) */
#define _O_U16TEXT      0x20000 /* file mode is UTF16 no BOM (translat
+ed) */
#define _O_U8TEXT       0x40000 /* file mode is UTF8  no BOM (translat
+ed) */
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So, assuming the variable $mode contains the OR'd values; you can check which bits are set something like:

if( $mode & 1 ) { ## _O_WRONLY
    say "File is opend WRITE only
}
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But note: not all of the bits that can be set when opening a file are retained in FILE*->_flag.

Thumbs up for this load of code!! Thank you BrowserUk!

As I remember, I "discovered" them by writing a small C program that open files in each of the modes and then queried the numeric value using this code (from Fmode.xs):

int 
xs_fmode( FILE *stream ) {
    return stream->_flag;
}
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And I discovered that incantation by finding the struct FILE definition in the C header files.

NOTE: This was done using MSVC header files; YMMV on other platforms.

See also FileHandle::Fmode and Fcntl

#!/usr/bin/perl --
use strict; use warnings; use Data::Dump;
use Fcntl();
my %mode2symbol = (
#~     Fcntl::F_DUPFD()  => '&',
    Fcntl::O_RDONLY() => '<',
    Fcntl::O_RDWR()   => '+<',
    Fcntl::O_TRUNC()  => '>',
    Fcntl::O_WRONLY() => '>',
    Fcntl::O_APPEND() => '>>',
    Fcntl::O_CREAT()  => '>',
);

my %name2mode = (
#~     F_DUPFD  => Fcntl::F_DUPFD(),
    O_RDONLY => Fcntl::O_RDONLY(),
    O_RDWR   => Fcntl::O_RDWR(),
    O_TRUNC  => Fcntl::O_TRUNC(),
    O_WRONLY => Fcntl::O_WRONLY(),
    O_APPEND => Fcntl::O_APPEND(),
    O_CREAT  => Fcntl::O_CREAT(),
);

$mode2symbol{ $name2mode{O_RDWR} | $name2mode{O_APPEND} } = '+>>';
$mode2symbol{ $name2mode{O_RDONLY} | $name2mode{O_APPEND} } = '+>>';
$mode2symbol{ $name2mode{O_WRONLY} | $name2mode{O_APPEND} } = '+>>';

dd \%mode2symbol;
dd \%name2mode;
__END__
{
  "0"   => "<",
  "1"   => ">",
  "2"   => "+<",
  "8"   => "+>>",
  "9"   => "+>>",
  "10"  => "+>>",
  "256" => ">",
  "512" => ">",
}
{
  O_APPEND => 8,
  O_CREAT  => 256,
  O_RDONLY => 0,
  O_RDWR   => 2,
  O_TRUNC  => 512,
  O_WRONLY => 1,
}
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