my $year_left = ...;
my $year_right = ...;

my $have_years = ($year_left and $year_right);
my $equal_years = ($year_left == $year_right);

my $five_words_result = ...; # you already have this above

my $final_result;

if( ! $have_years ) {
    $final_result = $five_words_result;
} elsif( $equal_years ) {
    $final_result = $five_words_result;
} elsif( ! $equal_years ) {
    $final_result = undef; # without matching years, things are never equal
} else {
    die "This should never happen. left=$year_left, right=$year_right, have_years=$have_years, equal_years=$equal_years";
};