As a follow-up to this, I would note that I have now got this working - The problem appeared to be related, not to the PerlTransHandler specification line, but rather the PerlModule line which would load in the perl module providing the PerlTransHandler handler. That is, uncommenting the first line in the virtual host configuration below (for virtual host B) results in the errors outlined above being returned through the SOAP::Lite interface (provided by virtual host A):

#    PerlModule TrustedDelivery::Redirect
    PerlTransHandler TrustedDelivery::Redirect
    PerlSetVar session_redirect /session.php
    PerlSetVar session_args _session=%s
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I am however at a loss to explain this observation - If there is interest, I am happy to post the source to the minimal TrustedDelivery::Redirect currently being employed for review and follow-up such that the underlying issue can be identified (and potentially corrected if this is an issue outside the scope of my module).