Does that take care of the "in front of an 'a'" requirement?

Er, no. I missed that bit of the spec, but it is easily corrected:

$s = 'bbaaccbab sdbcbalsbadcbnw';; ## with added bs

($t = $s) =~ s[(b)(?=a[^\s]+$)][B]g; print $t;;
bbaaccbab sdbcBalsBadcbnw
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