Perl Pocket Reference, 5th Edition

Smartmatching

Note: Smartmatching is a relatively new feature and several aspects of its peculiar behavior may change in subsequent Perl releases.

expr1 ~~ expr2

Smartmatches expr2 against expr1.

when ( expr )

In most cases, smartmatches $_ against expr.

In other cases, treats expr as a boolean expression.

The behavior of a smartmatch depends on what type of thing its arguments are. The behavior is determined by the following table: the first row that applies determines the match behavior (which is thus mostly determined by the type of the right operand). Note that the smartmatch implicitly dereferences any nonblessed hash or array ref, so the Hash and Array entries apply in those cases. For blessed references, the Object entries apply.

Left	Right	Type of match implied
Any	undef	Undefinedness.
Any	Object	Invokes ~~ overloading on the object, or dies.
Hash	CodeRef	Sub truth for each key; true if hash is empty.
Array	CodeRef	Sub truth for each element; true if array is empty.
Any	CodeRef	Scalar sub truth.
Hash	Hash	True if every key is found in both hashes.
Array	Hash	Hash keys intersection.
Regex	Hash	Hash key grep.
undef	Hash	Always false.
Any	Hash	Hash entry existence.
Hash	Array	Hash Keys Intersection.
Array	Array	Smartmatches corresponding elements.
Regex	Array	Array grep.
undef	Array	Array contains undef.
Any	Array	Match against an array element.
Hash	Regex	Hash key grep.
Array	Regex	Array grep.
Any	Regex	Pattern match.
Object	Any	Invokes ~~ overloading on the object, or falls back.
Any	Num	Numeric equality.
Num	Num-alike	Numeric equality if it ...