Update: Please use strict; !

Though I have not used this code, I have one comment to make in the spirit of TIMTOWTDI (hopefully a good comment) - I find the simulated switch statement to map a keypress to an action rather clumsy, as it does not really allow to easily add other actions later or add other actions dynamically.

In such situations, I prefer to use a hash of code references, like this :

my %actions = (
 "n" => \&skip,
 "m" => \&movesong,
# ... rest ommited ...
 "\n" => \&goto,
);
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and the routine interpretplay() would then look like this (sadly, untested) :

sub interpretplay {
  if (exists $actions{$char}) {
    &$actions{$char};
  } else {
    &invalid;
  };
};
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or, if the whole state machine would be changed to use hashes, one could even use a single interpret() method that would simply look at the $current_state hash reference to find the action to be executed.

Dynamically loaded actions would then simply modify the %actions hash with their entry.