If you use perl 5.10 or newer, you can use \K to exclude everything on its left from the subsitution: 
1 while s/ \s [^b]* \K b (?=a) /B/x;
#                    ^ only start the substitution here
#             ^ go to the first b
#          ^ only search second string

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(Update: added the "before a" criterion)

Perl 6 - links to (nearly) everything that is Perl 6.

In reply to Re: Work-around for variable length look-behind? by moritz
in thread Work-around for variable length look-behind? by pat_mc

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