I would first create a package that inherits DBIx:RetryOverdisconnect and then override any mortal methods with a routine that applies Moritz's eval solution e.g.

package DBIx::RetryOverDisconnects::Immortal;
use DBIx::RetryOverDisconnects;
sub DBIx::RetryOverDisconnects::Immortal::txn_do {
    my ($self, $coderef) = @_;
    my $success = 0;
    eval( $success = $self ->DBIx::RetryOverDisconnects::txn_do($coder
+ef) );
    $success;
}
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The use of this kind of module should obviate the need to recode the CPAN module.