You can't use IO::Scalar, but you can use PerlIO::scalar in 5.8.x:

format FOO=
@||| @||| @|||
$x,$y,$z
.
$x=1;$y=2;$z=3;
$save = "";
open FOO, ">", \$save;
write FOO;
print "got:$save!\n";
[download]

Reopening STDOUT is a little more complicated, but easily doable.