Description:

'PerlIO::tee' provides a multiplex output stream like 'tee(1)'. It makes a
filehandle write to one or more files (or scalars via the ':scalar' layer)
at the same time.

You can use 'push_layer()' (defined in 'PerlIO::Util') to add a _source_ to
a filehandle. The _source_ may be a file name, a scalar reference, or a
filehandle. For example:

$fh->push_layer(tee => $file); # meaning "> $file"
$fh->push_layer(tee => ">>$file");# append mode
$fh->push_layer(tee => \$scalar); # via :scalar
$fh->push_layer(tee => \*OUT); # shallow copy, not duplication