Ok, usual problem is that some characters in $CbBlkTx are regex special chars. If you use

$xmlBody =~ s/\Q$CbBlkTx\E/$bBlkTx/xgi;
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instead, all special characters between \Q and \E are escaped. If you want more information try 'perldoc -f quotemeta', quotemeta is the function behind \Q\E

UPDATE: Added missing word "information". I didn't want to insinuate that quotemeta delivers more escapes than \Q\E ;-)