sub promptTill{{ 
    local $\;
    print "$_[0]: "; 
    chomp( my $input = <STDIN> ); 
    redo unless $input =~ $_[1]; 
    return $input 
}}

print promptTill( 'Yes/No', qr[^(?:y(?:e(?:s)?)?|n(?:o)?)$]i );;