The synopsis of Mail::DeliveryStatus::BounceParser tells me that it wants each mail fed to it separately. As you have a file in mbox format containing several mails, you'll have to split up that file first, and then pass the single mails to Mail::DeliveryStatus::BounceParser like this, according to the synopsis:

use strict;
use Mail::DeliveryStatus::BounceParser;

my $mail; # current mail

while (<>) {
  if (/^From:/ && $mail) {
    my $bounce = eval { Mail::DeliveryStatus::BounceParser->new( $mail
+ ) };
    if ($@) { 
      warn "Got error from M::DS::BP: $@ while parsing input" 
    } else {
      my       @addresses = $bounce->addresses;       
      my         @reports = $bounce->reports;         
      my $orig_message_id = $bounce->orig_message_id;
      my $orig_message    = $bounce->orig_message;
      print "Got bounce for $orig_message_id (@reports)";
    };
    $mail = "";
  };
  $mail .= $_;
}
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That's all there is to it. The code is untested but should work like that.