If it's homework, by all means, figure out the logic. If not:

use IO::Prompt::Hooked;

my $choice = prompt(
  message  => 'Do you wish to update? (Y/N):',
  validate => qr/^[YN]$/i,
);
[download]

Or if you want to provide extra help upon receiving invalid input:

my $choice = prompt(
  message  => 'Do you wish to update? (Y/N):',
  validate => qr/^[YN]$/i,
  error    => "Response must be 'Y' or 'N'.\n\n",
);
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...or if you want to add a default...

my $choice = prompt(
  message  => 'Do you wish to update? (Y/N):',
  default  => 'N',
  validate => qr/^[YN]$/i,
  error    => "Response must be 'Y' or 'N'.\n\n",
);
[download]