I like:

use strict;

my $ansflg = 0;
while (!$ansflg) {
    # No valid answer received yet
    print "Confirm [Y,n]: ";
    my $geninp = <STDIN>;
    chomp $geninp;
    if ($geninp =~ /^\s*$/) {
        # Blank answer means DEFAULT
        $geninp = 'Y';
    }

    my $genexp = quotemeta $geninp;
    if ("YES" =~ /^$genexp/i) {
        # Said YES
        print "Answer = YES\n";
        $ansflg = 1;
    } elsif ("NO" =~ /^$genexp/i) {
        # Said NO
        print "Answer = NO\n";
        $ansflg = 1;
    } else {
        # Said something else
        # Do nothing; repeat the prompt
    }
}

exit;
[download]

This permits upper- or lowercase, Y, YE, YES, or a blank answer to be taken as a YES, while N and NO mean NO, and invalid answers don't slide past.