16.4. Unquoted Hash Keys

Perl offers many shortcuts that can help the programmer, such as omitting the quote marks on some hash keys.

You can't omit the quote marks on every key since a hash key may be any arbitrary string. But keys are often simple. If the hash key only consists of 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.