sub is_abc {
    local $_ = shift || $_;
    $_ !~ /[^abc]/
}

sub is_ab {
    local $_ = shift || $_;
    $_ !~ /[^ab]/
}

##</code><code>##

sub has_odd_ab {
    local $_ = shift || $_;
    my $n = () = /ab/g;    # count matches
    1 & $n                 # odd?
}

##</code><code>##

sub odd_a {
    local $_ = shift || $_;
    1 & tr/a//;
}
sub odd_b {
    local $_ = shift || $_;
    1 & tr/b//;
}

##</code><code>##

sub prob_one {
    local $_ = shift || $_;
    is_abc and has_odd_ab;
}
sub prob_two {
    local $_ = shift || $_;
    is_ab and ! odd_a or odd_b;
}