-C [number/list] The -C flag controls some of the Perl Unicode features. As of 5.8.1, the -C can be followed either by a number or a list of option letters. The letters, their numeric values, and effects are as follows; listing the letters is equal to summing the numbers. I 1 STDIN is assumed to be in UTF-8 O 2 STDOUT will be in UTF-8 E 4 STDERR will be in UTF-8 S 7 I + O + E i 8 UTF-8 is the default PerlIO layer for input streams o 16 UTF-8 is the default PerlIO layer for output streams D 24 i + o A 32 the @ARGV elements are expected to be strings encoded in UTF-8 L 64 normally the "IOEioA" are unconditional, the L makes them conditional on the locale environment variables (the LC_ALL, LC_TYPE, and LANG, in the order of decreasing precedence) -- if the variables indicate UTF-8, then the selected "IOEioA" are in effect a 256 Set ${^UTF8CACHE} to -1, to run the UTF-8 caching code in debugging mode. For example, -COE and -C6 will both turn on UTF-8-ness on both STDOUT and STDERR. Repeating letters is just redundant, not cumulative nor toggling. The io options mean that any subsequent open() (or similar I/O operations) will have the :utf8 PerlIO layer implicitly applied to them, in other words, UTF-8 is expected from any input stream, and UTF-8 is produced to any output stream. This is just the default, with explicit layers in open() and with binmode() one can manipulate streams as usual. -C on its own (not followed by any number or option list), or the empty string "" for the PERL_UNICODE environment variable, has the same effect as -CSDL. In other words, the standard I/O handles and the default open() layer are UTF-8-fied but only if the locale environment variables indicate a UTF-8 locale. This behaviour follows the implicit (and problematic) UTF-8 behaviour of Perl 5.8.0.