I'm not really sure if I understand all of your requirements: in fact I find your specifications slightly sloppy and incomplete. But if I do, then I'd just do something like this:

#!/usr/bin/perl -l

use strict;
use warnings;

{
    my $state;
    sub fail () { $state++ }
    sub failed () { $state ? $state-- : $state }
}

LINE: while (<>) {
    chomp;
    my @chunks=split /_/;
    fail, next if @chunks < 2;

    my $oldch='';
    for (@chunks) {
        /^([a-z])([a-z])\d*$/ or
          fail, next LINE;
        my $diff=(ord $2) - (ord $1);
        fail, next LINE unless 
          ($diff == 0 or $diff == 1) and
          $oldch lt $_;
        $oldch=$_;
    }
} continue {
    print "`$_': ",
      failed ? 'fails!' : 'succeeds!';
}

__END__
[download]

UPDATE: I am greately ashamed to admit that the original code posted here contained an unncessary, clumsy chunk of code as for some reason I ignored a thing called 'transitive law' altogether... I updated the code above accordingly to remove at least that gross mistake. Incidentally I also changed C<le> to C<lt> which seems more appropriate.

Of course this may not be exactly what you're after, but I hope it's close enough that you can easily adapt it to your needs. Also this is intended to be a minimal example: you may want to give more informative output by e.g. making C<fail()> accept a message argument (and use it suitably somewhere).

Oh, and yes: at a certain point I used C<?:> (also) for its side effects, but I hope that in this case it's only a venial sin and that it will be forgiven...