One approach I've used for really simple languages is a command hash. The keys of the hash are commands and the value is a code ref that executes the command. This approach is really not very flexible, but it fills the niche below a full scale language implementation.

my %command = (
    foo => \&do_foo,
    bar => sub { print "doing bar with parameters: @_\n"; },
  # ...
);
[download]

If you decide you really need a language, rather than a set of commands, look for parsing modules on CPAN.

The best advice I ever got on this issue was to write some examples of how you would use the language if it worked like you want. Then, go make it look like that.

Without some idea of the complexity of the language you want to implement, it is hard to guess which tool would be the least effort.