Yet another way to do it:

use strict;
use warnings;

my %data;
my @reverse;

while (<DATA>) {
    chomp;
    my ($prefix, $suffix) = /^([A-Z0-9]+?)(\d+|.)$/;
    
    if (exists $data{$prefix}) {
        my $mapFunc = $suffix =~ /[A-Z]/ ? sub {ord $_[0]} : sub {$_[0
+]};
        
        for (@{$data{$prefix}}) {
            next if abs ($mapFunc->($suffix) - $mapFunc->($_)) != 1;
            print "$prefix$_;$prefix$suffix\n";
            push @reverse, "$prefix$suffix;$prefix$_\n";
        }
    }
    
    push @{$data{$prefix}}, $suffix;
}

print @reverse;

__DATA__
AAA30
BBC5
SHT12H
DAL33B
BBC49
AAA31
DAL33A
BBC6
SHT12G
BBC50
[download]

Prints:

AAA30;AAA31
DAL33B;DAL33A
BBC5;BBC6
SHT12H;SHT12G
BBC49;BBC50
AAA31;AAA30
DAL33A;DAL33B
BBC6;BBC5
SHT12G;SHT12H
BBC50;BBC49
[download]