Which version of IO::Prompter? I see test failures on at least one Windows system in some of the earlier versions. The most recent version (0.004005) seems to have a more successful smoke test record, but I see from its change log that the most recent version disables interactive testing on Windows platforms, so it's possible a problem is being missed by the tests.

IO::Prompt::Tiny is designed to be highly portable, as it's based on ExtUtils::MakeMaker's prompt, which must be portable. Your simple functionality would be perfectly fine with IO::Prompt::Tiny. IO::Prompt::Hooked is based on IO::Prompt::Tiny, and should be as portable, but might even simplify your logic a little:

use IO::Prompt::Hooked;

my $string = prompt (
  message  => 'Input a string:',
  validate => sub { q{} ne shift }
);
[download]

...or even...

my $string = prompt (
  message  => 'Input a string:',
  validate => qr/./
);
[download]

Do make sure that the environment variable PERL_MM_USE_DEFAULT is not set.