my $out;
Filter \*STDOUT, sub {
    $out = sprintf "%d: %s\n", 1, "@_";
    ()
};
print "Foo";
UnFilter \*STDOUT;
print $out eq "1: Foo\n" ? "ok 3\n" : "not ok 3\n";

##</code><code>##

sub Filter {
    my $fh = $_[0];
    tie *{ $fh }, __PACKAGE__, @_;
}

sub UnFilter {
    my $fh = shift;
    { local $^W = 0; untie *{ $fh } }
}