This code would check if an action correponding to $key is known, determine the action if it's not known, and in either case execute the action:

($actions{$key} ||= determine_action($key))->($key, $data);
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sub determine_action {
    my ($key) = @_
    if (it's first action) {
        return sub {
            my ($key, $data) = @_;
            do stuff;
        };
    elsif (it's second action) {
        return sub {
            my ($key, $data) = @_;
            do stuff;
        };
... etc...
}
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Instead of just evaling the code, wrap the code in an anonymous sub, thus capturing it so you can resuse it. So we have a routine called parse_to_actions that builds a bunch of lines of perl statements that need to be executed. Then we do this:

my %code_cache;
while (<>) {
   my $code=$code_cache{$_};
   unless ($code) {
      my @actions=parse_to_actions($_);
      $code=eval "sub { @actions }" 
               or die "$@ while evaling actions @actions ";  
      $code_cache{$_}=$code;
   }
   $code->();
}
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However when you consider that we can define parse_to_actions to return a sub, then we could

use Memoize;

sub parse_to_actions {
  return eval "sub { @lines_of_code }" or die $@;
}

memoize("parse_to_actions");
while (<>) {
  #Parse and generate. Memoize caches.
  parse_to_actions($_)->($_); # pass the line to the generated sub 
                              # just in case it gets smart
}
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