use strict;
use warnings;
use Benchmark;

my $string = '.' x 10000;

sub tr_test {
	$string =~ tr/./_/;
	$string =~ tr/_/./;
}

sub s_test {
	$string =~ s/\./_/g;
	$string =~ s/_/./g;
}

my $count = 5000;

timethese ( $count, {
	'TR' =>	\&tr_test,
	'S'  => \&s_test
} );

__OUTPUT__
Benchmark: timing 5000 iterations of S, TR...
    S: 48 wallclock secs (46.95 usr +  0.01 sys = 46.96 CPU) 
        @  106.48/s (n=5000)
    TR: 0 wallclock secs ( 0.58 usr +  0.00 sys =  0.58 CPU) 
        @ 8605.85/s (n=5000)