0123456789
89a
bcdefghijklmn

##</code><code>##

use warnings;
use strict;

use Benchmark;

my $text   = "0123456789abcdefghijklmn";
my $unpack = "";
my $position = 0;


my @fields = 
(
  { name => 'name1', start =>  0, len=> 10 }, 
  { name => 'name2', start =>  8, len=>  3 },
  { name => 'name3', start => 11, len=> 14 },
);

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

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

print "$unpack\n";
print join ("*", unpack ($unpack, $text)), "\n";

timethese
(
	1_000_000,
	{   
		'unpack' => sub 
	   	{
			my @a = unpack ($unpack, $text);
		},
		'substr' => sub
		{
			my @a = map  { substr $text, $_->{start}, $_->{len} } @fields;
		}
	},
);

##</code><code>##

A10X2A3A14
0123456789*89a*bcdefghijklmn
Benchmark: timing 1000000 iterations of substr, unpack...
    substr: 12 wallclock secs (12.80 usr + -0.02 sys = 12.78 CPU) @ 78241.14/s (n=1000000)
    unpack:  9 wallclock secs ( 9.00 usr +  0.00 sys =  9.00 CPU) @ 111098.77/s (n=1000000)