Opens the file whose filename is given by FILENAME, and associates it with FILEHANDLE. If FILEHANDLE is an expression, its value is used as the name of the real filehandle wanted. This function calls the underlying operating system's open() function with the parameters FILENAME, MODE, PERMS.
The possible values and flag bits of the MODE parameter are system-dependent; they are available via the standard module Fcntl. For historical reasons, some values work on almost every system supported by perl: zero means read-only, one means write-only, and two means read/write. We know that these values do not work under OS/390 & VM/ESA Unix and on the Macintosh; you probably don't want to use them in new code.
If the file named by FILENAME does not exist and the open() call creates it (typically because MODE includes the O_CREAT flag), then the value of PERMS specifies the permissions of the newly created file. If you omit the PERMS argument to sysopen(), Perl uses the octal value 0666. These permission values need to be in octal, and are modified by your process's current umask.
You should seldom if ever use 0644 as argument to sysopen(), because that takes away the user's option to have a more permissive umask. Better to omit it. See the perlfunc(1) entry on umask for more on this.
See perlopentut for a kinder, gentler explanation of opening files.