Untested:

my $string = "abcd-ef1";
my @matches = $string =~ /^(\w{2})?(\d)?(\w{2})?(-)?(\w{2})?(\d)?$/;
my $errors = grep { !defined $_ } @matches;
print "Errors is $errors\n";
----
Errors is 1
[download]

Basically, you're expecting everything to match. If it doesn't, then non-matches are errors. You'll have to fiddle with it, I think, to get it to do exactly what you want, but that should give you a good start. (This is, of course, that you have to (re)invent the wheel.)

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We are the carpenters and bricklayers of the Information Age.

The idea is a little like C++ templates, except not quite so brain-meltingly complicated. -- TheDamian, Exegesis 6

Please remember that I'm crufty and crochety. All opinions are purely mine and all code is untested, unless otherwise specified.