That's a good description: b not followed by a string containing a b. You could also do it with "sexeger" (using a regex on the reverse of the string):

$string = reverse $string;
$string =~ s/b/x/;
$string = reverse $string;
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though that seems a little excessive for this case. The other usual way to do it is:

$string =~ s/(.*)b/$1x/;
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or (at least as clunky as your original):

$string =~ m/.*(?=b)/g and $string =~ s/\Gb/x/;
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Finally, you can do it without using a regex at all:

substr($string, rindex($string, 'b'), 1) = 'x';
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