> perl -MIO::Tee -e 'my $tee = IO::Tee->new(\*STDOUT,\*STDERR); print $tee "Test\n";'
Test
Test

> perl -MIO::Tee -e 'my $tee = IO::Tee->new(\*STDOUT,\*STDERR,\*STDOUT,\*STDERR); printf $tee "Test\n";'
Test
Test
Test
Test