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my $out;
Filter \*STDOUT, sub {
...
print "Foo";
UnFilter \*STDOUT;
print $out eq "1: Foo\n" ? "ok 3\n" : "not ok 3\n";

sub Filter {
    my $fh = $_[0];
...
    my $fh = shift;
    { local $^W = 0; untie *{ $fh } }
}