It would be a lot easier to help if you told us what you were trying to achieve instead of just giving us non-working code and expecting us to work it out for ourselves.

Also, adding use strict and use warnings to your code will help Perl to help you find the problems.

I'm guessing, but is this something like what you wanted?

#! /usr/bin/perl

use strict;
use warnings;

my @a;
my $b = "abefcdghijklmn" ;

$a[0]->[0] = "start 0" ;
$a[1]->[0] = "start 1" ;

for(my $i = 0; $i < 14; $i += 2) {
    my ($foo, $bar) = unpack("x$i A1 A1", $b) ;
    push @{$a[0]}, $foo;
    push @{$a[1]}, $bar;
}

foreach ( @{$a[0]} ) {
    print "$_\n" ;
}
[download]

If that is what you want, then it's probably more maintainable if it's written something like this:

#! /usr/bin/perl

use strict;
use warnings;

my @a;
my $b = "abefcdghijklmn" ;

$a[0]->[0] = "start 0" ;
$a[1]->[0] = "start 1" ;

my @chars = split //, $b;

while (@chars) {
  push @{$a[0]}, shift @chars;
  push @{$a[1]}, shift @chars;
}

foreach ( @{$a[0]} ) {
    print "$_\n" ;
}
[download]