use IO::Prompt::Hooked qw(prompt);

my $input = prompt(
  message  => 'Please enter 47:',
  validate => qr/^47$/,
);
print "You entered: $input\n";
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Sample run:

Please enter 47: a
Please enter 47: b
Please enter 47: 42
Please enter 47: 47
You entered: 47
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I feel it coming.... "But I can't use modules.":

• Why not?
• Then why are you using Perl?
• Then you should look at the fairly trivial source code for IO::Prompt::Hooked and see how it works. Or at minimum, read and comprehend perlintro.