Priority in Mail::Sender is just a header field 'X-Priority' with values from 1 to 5. So all you need is to set a header field: 
# Email::Stuffer
$email->header( 'X-Priority' => 1 );

# Email::Simple
$email->header_set( 'X-Priority' => 1 );

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In reply to Re: Mail::Sender code change by tangent
in thread Mail::Sender code change by Anonymous Monk

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