From type/Failure

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method handled(Failure:D: --> Bool:D) is rw

Returns True for handled failures, False otherwise.

sub f() { fail }; my $v = f; say $v.handled; # OUTPUT: «False␤»

The handled method is an lvalue, see routine trait is rw, which means you can also use it to set the handled state:

sub f() { fail }
my $v = f;
$v.handled = True;
say $v.handled; # OUTPUT: «True␤»