use strict;
use warnings;
use Benchmark qw(cmpthese);

my $x = "0123456789abcdefg" x 6000 . "XXX";

sub subst   { my $s = $x; $s =~ s/(.{16})/$1 /sg; $s };
sub up      { join ' ', unpack '(A16)*', $x }
sub m_substr {
    my $s = $x;
    for (my $i=16*int(length($s)/16);$i;$i-=16) { 
       substr($s,$i,0)=" "; 
    }
    $s;
}

sub readline {
    local $/ = \16;
    my $s;
    open my $handle, '<', \$x;
    $s = <$handle>;
    while (<$handle>) {
        $s .= ' ' . $_;
    }
    $s;
}

my $ref = subst();

sub wrap {
    my ($impl, $sub) = @_;
    die "Wrong result for $impl!" unless $sub->() eq $ref;
}

cmpthese(-1, {
    subst   => wrap('subst', \&subst),
    unpack  => wrap('unpack',\&up),
    substr  => wrap('substr',\&m_substr),
    readl   => wrap('readline', \&readline),
})

##</code><code>##

              Rate  readl  subst unpack substr
readl   78783885/s     --   -17%   -18%   -24%
subst   95302528/s    21%     --    -1%    -8%
unpack  96105165/s    22%     1%     --    -8%
substr 103929656/s    32%     9%     8%     --

##</code><code>##

              Rate  subst  readl substr unpack
subst   81695687/s     --    -1%   -36%   -65%
readl   82899185/s     1%     --   -35%   -65%
substr 128382089/s    57%    55%     --   -46%
unpack 235800076/s   189%   184%    84%     --