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);
    }
}