A solution using tye's Algorithm::Loops:

use Algorithm::Loops 'NestedLoops';

my %pat = (
  L => ['a' .. 'z'],
  N => [0 .. 9]
);

my $pattern = shift || "LLN";

NestedLoops(
  [ @pat{ split //, $pattern } ],
  sub { print join("", @_), $/ }
);
[download]

Or one using glob:

my %pat = (
  L => ['a' .. 'z'],
  N => [0 .. 9]
);

my $pattern = shift || "LLN";

$_ = sprintf "{%s}", join ",", @$_
  for values %pat;

my $glob = join "", @pat{ split //, $pattern };
print "$_\n" for glob($glob);
[download]

However, the glob version will generate all the expansions in memory (even if you use the iterator interface to glob). NestedLoops generates them as needed.

BTW, if your pattern calls for all 10-character-long strings, you will have between 10 billion and 100,000 billion strings generated. I'm guessing you probably don't need them all. What do you really want to accomplish?