Learning Perl, 5th Edition

Unquoted Hash Keys

Perl offers many shortcuts that can help programmers. Here’s a handy one: you may omit the quote marks on some hash keys.

Of course, you can’t omit the quote marks on just any key, since a hash key may be any arbitrary string. But keys are often simple. If the hash key is made up of nothing but letters, digits, and underscores without starting with a digit, you may be able to omit the quote marks. This kind of simple string without quote marks is called a bareword, since it stands alone without quotes.

One place you are permitted to use this shortcut is the most common place a hash key appears: in the curly braces of a hash element reference. For example, instead of $score{"fred"}, you could write simply $score{fred}. Since many hash keys are simple like this, not using quotes is a real convenience. But beware: if there’s anything inside the curly braces besides a bareword, Perl will interpret it as an expression.

Another place where hash keys appear is when assigning an entire hash using a list of key-value pairs. The big arrow (=>) is especially useful between a key and a value because (again, only if the key is a bareword) the big arrow quotes it for you:

# Hash containing bowling scores
my %score = (
  barney   => 195,
  fred     => 205,
  dino     => 30,
);

This is the one important difference between the big arrow and a comma; a bareword to the left of the big arrow is implicitly quoted. (Whatever is on the right is left alone, though.) You don’t have to use this feature of ...