Here's another benchmark.

use warnings;
use strict;
use Benchmark qw[ cmpthese ];

my @fields = (
    { name => 'name1', start =>  0, len=>  4 },
    { name => 'name2', start =>  3, len=>  7 },
    { name => 'name3', start =>  8, len=>  3 },
    { name => 'name4', start =>  0, len=> 10 },
    { name => 'name5', start =>  5, len=> 20 },
    { name => 'name6', start => 11, len=> 14 },
    { name => 'name7', start =>  9, len=> 13 },
    { name => 'name8', start =>  2, len=>  2 },
    { name => 'name9', start =>  1, len=> 10 },
);

open FH, '<', $ARGV[ 0 ] or die $!;

cmpthese -3, {
    'unpack' => sub {
    my $unpack = "";
    my $position = 0;

    for ( @fields ) {
        $unpack .= $_->{start} < $position ? 'X' . ( $position   - $_-
+>{start} ) :
                   $_->{start} > $position ? 'x' . ( $_->{start} - $po
+sition   ) :
                   '';

        $unpack .= 'A' . $_->{len};

        $position = $_->{start}+$_->{len};
    }
        seek FH, 0, 0;
        while( my $text = <FH> ) {
            my @a = unpack $unpack, $text;
        }
    },
    'substr' => sub {
        seek FH, 0, 0;
        while( my $text = <FH> ) {
            my @a = map{ substr $text, $_->{start}, $_->{len} } @field
+s;
        }
    }
};

close FH;
__END__
C:\test>for /l %i in (1,1,6) do holli data\alpha.1e%i

C:\test>holli data\alpha.1e1
           Rate unpack substr
unpack  28603/s     --   -80%
substr 140302/s   391%     --

C:\test>holli data\alpha.1e2
        Rate substr unpack
substr 400/s     --   -26%
unpack 540/s    35%     --

C:\test>holli data\alpha.1e3
         Rate substr unpack
substr 40.3/s     --   -27%
unpack 54.9/s    36%     --

C:\test>holli data\alpha.1e4
         Rate substr unpack
substr 4.06/s     --   -28%
unpack 5.61/s    38%     --

C:\test>holli data\alpha.1e5
            (warning: too few iterations for a reliable count)
            (warning: too few iterations for a reliable count)
       s/iter substr unpack
substr   2.47     --   -27%
unpack   1.80    37%     --

C:\test>holli data\alpha.1e6
            (warning: too few iterations for a reliable count)
            (warning: too few iterations for a reliable count)
       s/iter substr unpack
substr   25.3     --   -29%
unpack   18.0    40%     --
[download]

If your files are bigger than a few lines, the unpack version starts winning quite quickly.

But by nowhere near as much as the other benchmarks would have you believe because once you factor in reading each line from the file, the IO time which is constant for both approaches, the parsing time becomes much less significant. Worth having, but much less than you thought.