C:\test>1046579 -N=2**20
1048576
            Rate    single      long   classic       str translate
single    1.72/s        --      -99%     -100%     -100%     -100%
long       134/s     7707%        --      -71%      -79%      -81%
classic    468/s    27047%      248%        --      -26%      -33%
str        632/s    36579%      370%       35%        --       -9%
translate  695/s    40233%      417%       49%       10%        --

C:\test>1046579 -N=2**30
1073741824
            (warning: too few iterations for a reliable count)

            (warning: too few iterations for a reliable count)

          s/iter       str translate
str         1.96        --      -21%
translate   1.54       27%        --

##</code><code>##

#! perl -slw
use strict;
use Benchmark qw(cmpthese);

sub single {
    ${ $_[0] } =~ s/(.)/~$1/ges;
}

sub long {
    ${ $_[0] } =~ s/(.{1,10000})/~$1/ges;
}

sub classic {
    ${ $_[0] } = ~${ $_[0] };
}

sub str {
    my $blocksize = 10000;
    my $lb = length ${ $_[0] };
    my $offset = 0;
    while( $offset < $lb ) {
        substr ${ $_[0] }, $offset, $blocksize,
          ~substr( ${ $_[0] }, $offset, $blocksize );
        $offset += $blocksize;
    }
}

BEGIN {
    my $rev   = join '', map sprintf( "\\x%02x", $_ ), reverse 0 .. 255;
    eval "sub translate { \${ \$_[0] } =~ tr[\\x00-\\xff][$rev] } ; 1 " or die $@;
}

our $N //= 256; $N = eval "0+$N"; #print $N;
our $bits = pack 'C*', 0 .. 255;
$bits x= ( $N / 256 );

cmpthese -3, {
#    classic   => q[ classic(   \$bits)  ],
#    single    => q[ single(    \$bits ) ],
#    long      => q[ long(      \$bits ) ],
    translate => q[ translate( \$bits ) ],
    str       => q[ str(       \$bits ) ],
};