Getopt::Plus is a Perl module with options wrapper with standard options, help system and more.
Map from file size in bytes to human name, as hashref, keys being name (full name, lowercase, no trailing 's') and abbrev (one/two-letter abbreviation).
Permissable values to the type field of an option specifier.
Not an error at all. Hence the name.
Not so much an error as a non-erroneous circumstance worthy of signalling, e.g., grep finding no matches.
Again, not really an error, rather a utility function being called --- e.g., the --help or --version. This gets an error code because it is almost certainly an error to call from batch scripts.
The program was called wrong.
Some problem reading from disk or network (system read).
Some problem writing to disk or network (system write).
Some problem reading from db or similar (application read).
Some problem writing to db or similar (application write).
Some problem with an external application.
An internal logic error (the sort of thing that should never happen, but has been caught by an internal assertion or sanity check).
Some problem with the input file (which was read fine, but contains bad data).