As a partial answer to your problem, here are two subroutines which give you the previous and next values:

use strict;

my $plus;
my $minus;
while (<DATA>) {
    chomp;
    my $next     = next_seq($_);
    my $previous = previous_seq($_);
    print "$_, $next, $previous\n";
}

sub next_seq {
    my $pattern = shift;
    chomp $pattern;
    if ($pattern =~ m/(.*?)(\d+)$/) {
        return $1 . ($2 + 1);
    }
    else {
        $pattern =~ m/(.*)(.)/;
        return $1 . chr(ord($2) + 1);
    }
}

sub previous_seq {
    my $pattern = shift;
    chomp $pattern;
    if ($pattern =~ m/(.*?)(\d+)$/) {
        return $1 . ($2 - 1);
    }
    else {
        $pattern =~ m/(.*)(.)/;
        return $1 . chr(ord($2) - 1);
    }
}

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

Warning: I'm not sure it will work with unicode-strings and anyhow it will only take into account the last character (unless the pattern ends in a one or more digits). So @ precedes A and [ follows Z.

Now all you have to do is go through the list and search for items which match the next or previous values.