use strict;
use warnings;
use Mail::Mailer;
package Mail::Mailer;
my $basic_new;
BEGIN{
    $basic_new = *Mail::Mailer::new{CODE};
}
sub new($@){
    my @args = @_;
    $args[1] = 'testfile';# override type passed in
    use Data::Dumper;
    print Dumper \@args;
    return $basic_new->(@args);
}

package main;

my $m = Mail::Mailer->new('smtp');
$m->open({To => 'me@here.com'});
print $m "hi\n";