Do you really mean '$pw' with single quotes? (no quotes should be necessary at all) Also, I've never actually used Convert::BER at this level, I thought Net::LDAP automatically converted strings in the replace() data as necessary. ie. sth like:

$ldaps->modify(dn => $tempDN,
               changes => [ replace =>[unicodePwd => $pw]]);
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Also, I've tended to fetch an entry, call its replace/add methods and then update() it, but that should just fall through to this method...