+=item :unix
+
+Lowest level layer which provides basic PerlIO operations in terms of
+UNIX/POSIX numeric file descriptor calls
+(open(), read(), write(), lseek(), close()).
+
+=item :stdio
+
+Layer which calls C<fread>, C<fwrite> and C<fseek>/C<ftell> etc. Note
+that as this is "real" stdio it will ignore any layers beneath it and
+go straight to the operating system via the C library as usual.
+
+=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 :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. Note that this layer likes to be
+one of its kind: it silently ignores attempts to be pushed into the
+layer stack more than once.
+
+It currently does I<not> mimic MS-DOS as far as treating of Control-Z
+as being an end-of-file marker.
+
+(Gory details follow) To be more exact what happens is this: after
+pushing itself to the stack, the C<:crlf> layer checks all the layers
+below itself to find the first layer that is capable of being a CRLF
+layer but is not yet enabled to be a CRLF layer. If it finds such a
+layer, it enables the CRLFness of that other deeper layer, and then
+pops itself off the stack. If not, fine, use the one we just pushed.
+
+The end result is that a C<:crlf> means "please enable the first CRLF
+layer you can find, and if you can't find one, here would be a good
+spot to place a new one."
+
+Based on the C<:perlio> layer.
+
+=item :mmap
+
+A layer which implements "reading" of files by using C<mmap()> to
+make a (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 the 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 the platform does not support C<mmap()>.
+
+=item :utf8
+
+Declares that the stream accepts perl's I<internal> encoding of
+characters. (Which really is UTF-8 on ASCII machines, but is
+UTF-EBCDIC on EBCDIC machines.) This allows any character perl can
+represent to be read from or written to the stream. The UTF-X encoding
+is chosen to render simple text parts (i.e. non-accented letters,
+digits and common punctuation) human readable in the encoded file.
+
+Here is how to write your native data out using UTF-8 (or UTF-EBCDIC)
+and then read it back in.
+
+ open(F, ">:utf8", "data.utf");
+ print F $out;
+ close(F);
+
+ open(F, "<:utf8", "data.utf");
+ $in = <F>;
+ close(F);
+
+Note that this layer does not validate byte sequences. For reading
+input, using C<:encoding(utf8)> instead of bare C<:utf8> is strongly
+recommended.
+
+=item :bytes
+
+This is the inverse of the C<:utf8> layer. It turns off the flag
+on the layer below so that data read from it is considered to
+be "octets" i.e. characters in the range 0..255 only. Likewise
+on output perl will warn if a "wide" character is written
+to a such a stream.
+
+=item :raw
+
+The C<:raw> layer is I<defined> as being identical to calling
+C<binmode($fh)> - the stream is made suitable for passing binary data,
+i.e. each byte is passed as-is. The stream will still be
+buffered.
+
+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 the 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 the PERLIO environment variable.