in reply to Multiple replacements with the exception of previously made replacements.

Is there any way to achieve this with a single s/// operation?

How about with zero s/// operations? (Use tr/// instead :) 

[0] Perl> $s = 'x x x a x x x b x x x a x x x b x x x';;
[0] Perl> $s =~ tr[ab][ba];;
[0] Perl> print $s;;
x x x b x x x a x x x b x x x a x x x

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