Are you sure that's the error you got? I can't find any mention of it in perldiag.

No, prototypes are not required. In fact, prototypes are not required on forward-declerations even if the sub has a prototype (although you run the risk of having the prototype silently ignored).

use strict;
use warnings;

sub test1;
sub test2;
sub test3($$);

test1;
test2;            # No prototype at this time.
test3 'a', 'b';   # Args required by prototype.

sub test1     { print "test1\n"; }
sub test2($$) { print "test2\n"; }
sub test3($$) { print "test3\n"; }

test1;
test2 'a', 'b';   # Args required by prototype.
test3 'a', 'b';   # Args required by prototype.
[download]