s/\n/\n/って無駄じゃない？ 一番遅いよ

use strict;
use Benchmark;
$/=undef;
open H,'foo'; my $S=<H>; close H;
my $CNT=100;
bench( sub{ my $C = $S =~ tr/\n//;
$C++ if substr($S,length$S,1) ne "\n"; $C });
bench( sub {my $C = $S =~ s/\n/\n/g;
$C++ if substr($S,length$S,1) ne "\n"; $C});
bench( sub { my($i,$C);
while($i = index $S, "\n", $i+1) { $C++ }
$C++ if (substr($S,length$S,1) ne "\n"); $C});
sub bench {
my ($func, $C) = shift;
my $t0 = new Benchmark;
for(0..$CNT) { $C = $func->() }
my $t1 = new Benchmark;
my $td = timediff($t1, $t0);
print "$C ";
print "the code took:",timestr($td),"\n";
}