Update: It just struck me that using a hash for this is silly. String offsets are integers that run from 0!

#! perl -slw
use strict;
use Data::Dumper;

my @hash;
while( my $line = <DATA> ) {
    chomp $line;
    push @{ $hash[ $_ ] }, substr $line, $_, 2 
        for 0 .. length( $line ) -2;
}
print Dumper \@hash;

__DATA__
ABCDEFGHIJKLM
abcdefghijklm
NOPQRSTUVWXYZ
nopqrstuvwxyz
[download]

Ignore the code below.

#! perl -slw
use strict;
use Data::Dumper;

my %hash;
while( my $line = <DATA> ) {
    chomp $line;
    push @{ $hash{ $_ } }, substr $line, $_, 2 
        for 0 .. length( $line ) -2;
}
print Dumper \%hash;

__DATA__
ABCDEFGHIJKLM
abcdefghijklm
NOPQRSTUVWXYZ
nopqrstuvwxyz
[download]