my $first_type = $first_book =~ /^M/ ? 'major' : $first_book =~ /^m/  ? 'mentioned' : undef;

#could be coded as:
my $first_type = 'major'  if $first_book =~ /^M/;
$first_type = 'mentioned' if $first_book =~ /^m/;
$first_type //= 'unknown';