8.19.3. Discussion

You can easily specify I/O layers when you open a filehandle directly, but that doesn't help you when the filehandle is opened by someone else's code (possibly even the Perl core). The open pragma lets you specify a default set of layers for every open that doesn't specify its own layers.

The open module also offers separate IN and OUT control for input and output handles. For example, to read bytes and emit UTF-8:

use open "IN" => ":bytes", "OUT" => ":utf8";

The :std option tells open to apply the input and output layers to STDIN and STDOUT/STDERR. For example, the following code makes input handles read Greek (ISO 8859-7) and output handles write in the UTF-8 Unicode encoding. Then it applies the same layers to STDIN, STDOUT, and STDERR:

use open "IN" => ":encoding(Greek)",     # reading Greek
         "OUT" => ":utf8",               # writing 8-bit data in Unicode UTF-8,
         ":std";                         # STDIN is Greek,