One neat thing you can play with is a hash-based dispatch table. The idea is you store code references as the hash values, and look up the code by key, as necessary.

Here's a simplistic example that hopefully gives you some idea of the power behind these constructs. Say you have a string of text, and you want to read it word-by-word, reacting to certain words in different ways. A traditional way to do that would be:

my $text = "I am going to walk to the park.";

for ($text =~ /(\w+)/g) {
    if    (lc $_ eq "walk") { print "Walking is good for you\n"; }
    elsif (lc $_ eq "park") { print "Parks are fun\n"; }
}
[download]

That's good and well, but not very flexible, or scalable. Each new entry would require a new branch in the if/elsif chain, which can really start to add up. A hash-based approach might look something like this:

my %react_to = (
    walk => sub { print "Walking is good for you\n" },
    park => sub { print "Parks are fun\n" },
);

my $text = "I am going to walk to the park.";

for ($text =~ /(\w+)/g) {
    if (my $reaction = $react_to{lc $_}) {
        $reaction->();
    }
}
[download]

This one may seem more complicated, but it's moving the logic out of a nested chain of if/elsif statements and into a data structure. All sorts of neat things can then be done with that data structure, including modifying it at runtime. Here's a dispatch table that modifies itself at runtime:

my %react_to = (
    a => sub { shift->{b} = sub { print "A before B\n" } },
    b => sub { shift->{a} = sub { print "B before A\n" } },
);

my $text = "b is the first word, but a is not the last.";

for ($text =~ /(\w+)/g) {
    print "Read word: $_\n";

    if (my $reaction = $react_to{$_}) {
        $reaction->(\%react_to);
    }
}
[download]

This may be a contrived example, and it relies on the calling code passing the dispatch table in as an argument, but the idea is there.

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