+=item PERLIO
+
+A space (or colon) separated list of PerlIO layers. If perl is built
+to use PerlIO system for IO (the default) these layers effect perl's IO.
+
+It is conventional to start layer names with a colon e.g. C<:perlio> to
+emphasise their similarity to variable "attributes". But the code that parses
+layer specification strings (which is also used to decode the PERLIO
+environment variable) treats the colon as a separator.
+
+An unset or empty PERLIO is equivalent to C<:stdio>.
+
+The list becomes the default for I<all> perl's IO. Consequently only built-in
+layers can appear in this list, as external layers (such as :encoding()) need
+IO in order to load them!. See L<"open pragma"|open> for how to add external
+encodings as defaults.
+
+The layers that it makes sense to include in the PERLIO environment
+variable are summarised below. For more details see L<PerlIO>.
+
+=over 8
+
+=item :bytes
+
+A pseudolayer that turns I<off> the C<:utf8> flag for the layer below.
+Unlikely to be useful on its own in the global PERLIO environment variable.
+You perhaps were thinking of C<:crlf:bytes> or C<:perlio:bytes>.
+
+=item :crlf
+
+A layer that implements DOS/Windows like CRLF line endings.
+On read converts pairs of CR,LF to a single "\n" newline character.
+On write converts each "\n" to a CR,LF pair.
+Based on the C<:perlio> layer.
+
+=item :mmap
+
+A layer which implements "reading" of files by using C<mmap()> to
+make (whole) file appear in the process's address space, and then
+using that as PerlIO's "buffer". This I<may> be faster in certain
+circumstances for large files, and may result in less physical memory
+use when multiple processes are reading the same file.
+
+Files which are not C<mmap()>-able revert to behaving like the C<:perlio>
+layer. Writes also behave like C<:perlio> layer as C<mmap()> for write
+needs extra house-keeping (to extend the file) which negates any advantage.
+
+The C<:mmap> layer will not exist if platform does not support C<mmap()>.
+
+=item :perlio
+
+A from scratch implementation of buffering for PerlIO. Provides fast
+access to the buffer for C<sv_gets> which implements perl's readline/E<lt>E<gt>
+and in general attempts to minimize data copying.
+
+C<:perlio> will insert a C<:unix> layer below itself to do low level IO.
+
+=item :pop
+
+An experimental pseudolayer that removes the topmost layer.
+Use with the same care as is reserved for nitroglyserin.
+
+=item :raw
+
+A pseudolayer that manipulates other layers. Applying the <:raw>
+layer is equivalent to calling C<binmode($fh)>. It makes the stream
+pass each byte as-is without any translation. In particular CRLF
+translation, and/or :utf8 intuited from locale are disabled.
+
+Arranges for all accesses go straight to the lowest buffered layer provided
+by the configration. That is it strips off any layers above that layer.
+
+In Perl 5.6 and some books the C<:raw> layer (previously sometimes also
+referred to as a "discipline") is documented as the inverse of the
+C<:crlf> layer. That is no longer the case - other layers which would
+alter binary nature of the stream are also disabled. If you want UNIX
+line endings on a platform that normally does CRLF translation, but still
+want UTF-8 or encoding defaults the appropriate thing to do is to add
+C<:perlio> to PERLIO environment variable.
+
+=item :stdio
+
+This layer provides PerlIO interface by wrapping system's ANSI C "stdio"
+library calls. The layer provides both buffering and IO.
+Note that C<:stdio> layer does I<not> do CRLF translation even if that
+is platforms normal behaviour. You will need a C<:crlf> layer above it
+to do that.
+
+=item :unix
+
+Lowest level layer which provides basic PerlIO operations in terms of
+UNIX/POSIX numeric file descriptor calls
+C<open(), read(), write(), lseek(), close()>
+
+=item :utf8
+
+A pseudolayer that turns on a flag on the layer below to tell perl
+that data sent to the stream should be converted to perl internal
+"utf8" form and that data from the stream should be considered as so
+encoded. On ASCII based platforms the encoding is UTF-8 and on EBCDIC
+platforms UTF-EBCDIC. May be useful in PERLIO environment variable to
+make UTF-8 the default. (To turn off that behaviour use C<:bytes>
+layer.)
+
+=item :win32
+
+On Win32 platforms this I<experimental> layer uses native "handle" IO
+rather than unix-like numeric file descriptor layer. Known to be
+buggy in this release.
+
+=back
+
+On all platforms the default set of layers should give acceptable results.
+
+For UNIX platforms that will equivalent of "unix perlio" or "stdio".
+Configure is setup to prefer "stdio" implementation if system's library
+provides for fast access to the buffer, otherwise it uses the "unix perlio"
+implementation.
+
+On Win32 the default in this release is "unix crlf". Win32's "stdio"
+has a number of bugs/mis-features for perl IO which are somewhat
+C compiler vendor/version dependent. Using our own C<crlf> layer as
+the buffer avoids those issues and makes things more uniform.
+The C<crlf> layer provides CRLF to/from "\n" conversion as well as
+buffering.
+
+This release uses C<unix> as the bottom layer on Win32 and so still uses C
+compiler's numeric file descriptor routines. There is an experimental native
+C<win32> layer which is expected to be enhanced and should eventually replace
+the C<unix> layer.
+
+=item PERLIO_DEBUG
+
+If set to the name of a file or device then certain operations of PerlIO
+sub-system will be logged to that file (opened as append). Typical uses
+are UNIX:
+
+ PERLIO_DEBUG=/dev/tty perl script ...
+
+and Win32 approximate equivalent:
+
+ set PERLIO_DEBUG=CON
+ perl script ...
+
+
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