sub check {
    my ($name1, $name2) = @_;
    return unless length($name1) eq length($name2);
    my $letter_diff = 0;
    my $number_diff = 0;
    for (my $i = 0; $i < length($name1); $i++) {
        my $ch1 = substr $name1, $i, 1;
        my $ch2 = substr $name2, $i, 1;
        next if $ch1 eq $ch2;
        if ($ch1 =~ /\pL/ && $ch2 =~ /\pL/) {return if ++$letter_diff > 1; next}
        if ($ch1 =~ /\pN/ && $ch2 =~ /\pN/) {return if ++$number_diff > 1; next}
        return;  # Differ in non-letter/non-digits
    }
    return $letter_diff == 1 && $number_diff == 1;
}