Sequence expressions

The expression E1 , E2 returns the sequence obtained by concatenating the sequences E1 and E2.

For example, $x = ("London", "Paris", "Tokyo") returns true if the value of $x is one of the strings listed.

A sequence is often written in parentheses, as in the above example, but the parentheses are technically necessary only for operator precedence reasons.

The expression () denotes an empty sequence, which is often used to indicate absence of a value.