• Your deref $self->_getGood() twice, but the dump shows it's just a reference to an array of strings.
• Your map returns a list of file names, but you assign it to a scalar. That sets $files_to_send to 6, the number of file names.
  Your map would return a list of file names, but you assign the result to a scalar. This causes map to return 6, the number of file names.

sub _getGood {
    my ($self,@args) = @_;
    print "_getGood()", Dumper $self->{_good};

    # Returns a list of of 'Cxxxx' codes.
    return @{$self->{_good}};
}

my @files_to_send = map {"$stage/$_.pdf"} $self->_getGood();
print map {"$_\n"} @files_to_send;
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Update: The word "twice" was missing.
Update: Adjusted the wording for clarity in response to merlyn's reply.