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s/$c$a/$c$b/g;

or

s/($c)$a/$1$b/g;

    use Benchmark;
    use vars qw/$a $b $c/;
...
        lookbehind => sub { local $_ = "foobarquux"; s/(?<=$c)$a/$b/g 
+},
        chromatic  => sub { local $_ = "foobarquux"; s/($c)$a/$1$b/g  
+},
    });

    Benchmark: timing 500000 iterations of chromatic, lookbehind...
    chromatic: 15 wallclock secs (14.46 usr +  0.00 sys = 14.46 CPU)
    lookbehind: 13 wallclock secs (11.93 usr +  0.00 sys = 11.93 CPU)