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use Benchmark;

...
                'Regexp D&P'  => sub { $string =~ s/.(?!.{0,3}$)/x/g; 
+},
                'Substring'   => sub { $string = length substr($string
+,0,-4). substr($string,-4) }
              });

Benchmark: running Regexp, Regexp D&P, Substring, each for at least 5 
+CPU seconds...
    Regexp:  6 wallclock secs ( 5.38 usr +  0.00 sys =  5.38 CPU) @ 17
+3245.35/s (n=932060)
Regexp D&P:  6 wallclock secs ( 5.30 usr +  0.00 sys =  5.30 CPU) @ 20
+0480.57/s (n=1062547)
 Substring:  7 wallclock secs ( 5.45 usr +  0.00 sys =  5.45 CPU) @ 10
+04355.05/s (n=5473735)