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

Why not use the tr/y operator - described (here)? 
use warnings;
use strict;

my $test = 'x x x a x x x b x x x a x x x b x x x';
warn $test;

(my $res = $test) =~ y/[ab]/[ba]/;
warn $res;

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$  perl tst.pl
x x x a x x x b x x x a x x x b x x x at tst.pl line 5.
x x x b x x x a x x x b x x x a x x x at tst.pl line 8.

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A user level that continues to overstate my experience :-))

In Section Seekers of Perl Wisdom