Rather than

if ( not length $message->as_string
  or $message->as_string =~ /\A\s*\z/ )
{
    cluck "$in_filepath: Invalid message";
    next;
}
[download]

I would prefer something more along the lines of this:

my @errors;

length $message->as_string or
  push ( @errors, 'Message empty' );
$message->as_string =~ /\A\s*\z/ or
  push ( @errors, 'Invalid message' );

if ( @errors ) {

  foreach my $e ( @errors ) { cluck "in_filepath: $e"; }
  next;
}
[download]

Also, I would recommend || to or two conditionals together, rather than or.