OK, so I started using the logger object, which provided more information but nothing that I found useful. For the future people reading this the Log::Log4perl module works perfectly for this.

I went back over my code and switched from using the ticket->attachments pointer (which doesn't reference the undecoded option as far as I can tell) to the get_attachment_ids / get_attachment loop way. I also created a one line text file, zipped it up and added it to a new RT ticket on my system so that I had a 75 byte gzip file to test with. This worked, I was able to download, unzip and read the file, so I went back and tried the original test ticket, which also worked. I will post the working code below.

Having got this to work I went back over my old code to see what happened before and the answer is, as always, a simple one. Instead of using the undecoded => 1 parameter I had uudecoded => 1. This seemed reasonable to my eye as I scoured the code for errors because I Know Stuff (tm), I knew that uuen/decoding was a valid way of coding characters so didn't question it, it looked right.

So the fault was all mine, a simple typo, although I will add an RFE to the RT::Client::REST developers to add warnings if unknown options are specified as this would have saved me and you a few weeks worth of debugging. The final, working code looks like this: 

#!/usr/bin/env perl
#
# wctest.pl - A test to retrieve and save an attachment.

use strict;
use warnings;
use RT::Client::REST;
use RT::Client::REST::Ticket;
use Log::Log4perl;

my $user='xxxx';
my $pass='yyyy';

my $rt = RT::Client::REST->new(
    server  => ('https://rt.local'),
    basic_auth_cb => ( sub { return ($user, $pass); } ),
);

$rt->login( username=> $user, password=> $pass,);

#
# Get attachments using get_attachment
#

my @results = $rt->search( type => 'ticket', query => "id=51447" );

my ($id, @atch_ids, $atch_id, $atch);

for $id (@results) {
    @atch_ids = $rt->get_attachment_ids( id => $id);
    for $atch_id (@atch_ids) {
        $atch = $rt->get_attachment (parent_id => $id, id => $atch_id,
+ undecoded => 1);
        next if (! defined($atch->{'Filename'}) );
        next if ($atch->{'Filename'} eq '');
        open(FH, ">", $atch->{'Filename'}) || die "Can't open file: $!
+";
        syswrite(FH, $atch->{'Content'});
        close FH;
    }
}

[download]
Many thanks to you for your patience, help and assistance.

In reply to Re^3: RT::Client turns occasional binary characters in to wide characters by wardmw
in thread RT::Client turns occasional binary characters in to wide characters by wardmw

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