Expanding from chromatic's answer--as he said, Mail::Mailer inherits from IO::Handle, so a Mail::Mailer object is a filehandle.

You said you tried

select $mailer;
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Did that not work for you? It worked for me:

my $mailer = new Mail::Mailer 'smtp', 'relay.foo.com';
$mailer->open({
    To      => 'baz@foo.com',
    From    => 'foo@bar.com',
    Subject => 'Testing' });

select $mailer;
print "This is a test message.\n";

$mailer->close;
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