I originally got it into my head that setting the size for slurping was a good thing after reading Slurp-Eazy. I seem to recall that it seemed quicker on my old portable under 5.6.1.

However, looking again now under 5.8.4, the picture is both mixed and confusing:

(ordered both ways to eliminate the possibilty of "first run bias")

#! perl -slw
use strict;
use Benchmark qw[ cmpthese ];

open our $raw, '<:raw', $ARGV[ 0 ] or die $!;
open our $txt, '<    ', $ARGV[ 0 ] or die $!;

our $s = -s $raw;
cmpthese -3, {
    d_raw_trad => q[ my $data = do{ local $/;       <$raw> }; ],
    c_raw_size => q[ my $data = do{ local $/ = \$s; <$raw> }; ],
    b_txt_trad => q[ my $data = do{ local $/;       <$txt> }; ],
    a_txt_size => q[ my $data = do{ local $/ = \$s; <$txt> }; ],
};
__END__
[15:46:09.53] P:\test>440395 100.dat
             Rate raw_size raw_trad txt_trad txt_size
a_raw_size   7969/s       --     -87%     -88%     -96%
b_raw_trad  63136/s     692%       --      -2%     -69%
c_txt_trad  64282/s     707%       2%       --     -69%
d_txt_size 206315/s    2489%     227%     221%       --

[15:53:19.11] P:\test>440395 100.dat
               Rate c_raw_size d_raw_trad b_txt_trad a_txt_size
c_raw_size   7855/s         --       -87%       -88%       -96%
d_raw_trad  62625/s       697%         --        -4%       -70%
b_txt_trad  65166/s       730%         4%         --       -68%
a_txt_size 206449/s      2528%       230%       217%         --
[download]

Now quite why slurping a file in text-mode would be faster (and soo much faster!) than doing so in raw mode leaves me at a total loss for an explaination. Seems that I should reassess what I thought I once knew about Perl IO-layers and performance.