A method of class
UNIVERSAL (which see for notes on availability and use; basically it's available on all objects).
$x->can('foo') checks if
foo, that is whether
$x->foo is a legal method call (taking inheritance into account, of course).
can returns the code reference of the method which
$x->foo would call, or
$x can't (sorry)
foo. As such,
can is often used for this particular return value, to get a grip on the method's code reference.