use File::Find::Rule;

my $template = 'aaaa-nnnnnnnn';
(my $regex = $template)
  =~ s/([an])/$1 eq 'a' ? '[a-z]' : '[0-9]'/eig;

my $matcher = rule(
  file     =>
  maxdepth => 1,
  name     => qr/^$regex/i,
  start    => $ARGV[0]
);

while(my $file = $matcher->match) {
  ...
}