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.