The code runs fine on Ubuntu & perl 5.10.1 if you remove the pre-declared prototype (sub foo();). You don't need prototypes. They very probably don't do whatever it is you think they do.

Update: This works for me on 5.10.1. Try it on your 5.10.0:

#!perl510 -w

use strict;

#sub foo();
foo();

sub foo() { # shouldn't use () but does no harm here
  my $foo;

  format STDOUT =
^
$foo;
.

  $foo="X";
  write;
}
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I've been doing a lot of Python recently. Semi-colons sure do look weird now :)