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

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

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

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