Well, looking at Mail::IMAPClient.pm on CPAN I find that v3.08 contains:

756     # Get the "+ Go ahead" response:
757     my $code = 0;
758     until($code eq '+' || $code =~ /NO|BAD|OK/)
759     {
760          my $readSoFar  = 0;
761          my $fromBuffer = '';;
762          $readSoFar += sysread($toSock, $fromBuffer, 1, $readSoFar
+) || 0
763              until $fromBuffer =~ /\r\n/;
764
765          $code = $fromBuffer =~ /^\+/ ? $1
766                : $fromBuffer =~ / ^(?:\d+\s(BAD|NO))/ ? $1 : 0;
767
768          $peer->_debug("$folder: received $fromBuffer from server"
+);
769
770          # ... and log it in the history buffers
771          $self->_record($trans, [0, "OUTPUT",
772            "Mail::IMAPClient migrating message $mid to $peer->User
+\@$peer->Server"] );
773          $peer->_record($ptrans, [0, "OUTPUT", $fromBuffer] );
774     }
[download]

so, line 765 will set $code to undef if $fromBuffer does start with '+', which will generate the warnings you are seeing -- reported against the last (significant) line of the until because... because that's what it does.

The latest version is v3.11, and is similarly flawed. It is dated 08-Oct-2008, so at least it appears to be "live". Suggest you take this up with the author via http://search.cpan.org/~markov/Mail-IMAPClient-3.11/lib/Mail/IMAPClient.pod.