my %actions = (
  greet  => sub { print "Hello world!\n"; },
  growl  => sub { print "Rawwwrrrr!\n";   },
  depart => sub { print "Goodbye!\n";     },
  unrecognized => sub { warn "I don't know how to $_[0].\n"; },
);

while( <DATA> ) {
    chomp;
    if( exists $actions{$_} ) {
        $actions{$_}->();
    }
    else {
        $actions{unrecognized}->($_);
    }
}
__DATA__
greet
growl
whine
depart

##</code><code>##

Hello world!
Rawwwrrrr!
I don't know how to whine.
Goodbye!