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The first two arguments
($wtr and
$rdr) are passed in the
opposite order.

A third filehandle can be passed, for standard error. If this
argument is given as ", then
STDERR and STDOUT for $cmd are on the same
filehandle.

If $wtr begins with <, then
the leading < is stripped from the name, and
the remainder is assumed to be a regular filehandle for an open file,
rather than a reference to a typeglob. open3 opens
this file as STDIN for $cmd and closes it in the
parent. Likewise, if $rdr or
$err begins with >, then
$cmd directs STDOUT or STDERR directly to that
file rather than to the parent.