For more information on this, find yourself a Unix system and check out the sys/stat.h file (you may have to follow a chain of #includes on particularly strange OS's like Linux) but you're looking for a file that contains #defines like this:

#define __S_IFMT        0170000 /* These bits determine file type.  */

/* File types.  */
#define __S_IFDIR       0040000 /* Directory.  */
#define __S_IFCHR       0020000 /* Character device.  */
#define __S_IFBLK       0060000 /* Block device.  */
#define __S_IFREG       0100000 /* Regular file.  */
#define __S_IFIFO       0010000 /* FIFO.  */
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And so on. If you bitwise AND these definitions against that 6-digit octal number (st_mode), you'll see how the -X file tests work in Perl.