You need to dereference the reference. Just as when $a contains an array reference, @$a will dereference it to give the array, so when $c contains a code reference, &$c will dereference it to call the code. To pass arguments, you just add a parenthesised list in the normal fashion:

  sub sum {
    my($term, $a, $next, $b) = @_;
    return 0 if $a > $b;
    return &$term($a) + sum($term, &$next($a), $next, $b);
  }
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You can also use the dereferencing arrow, in which case the parens are required:

  sub sum {
    my($term, $a, $next, $b) = @_;
    return 0 if $a > $b;
    return $term->($a) + sum($term, $next->($a), $next, $b);
  }
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