From type/Mu

See Original text in context

method not()

Evaluates the item in Boolean context (leading to final evaluation of Junctions, for instance), and negates the result. It is the opposite of so and its behavior is equivalent to the ! operator.

my @args = <-a -e -b>;
my $verbose-selected = any(@args) eq '-v' | '-V';
if $verbose-selected.not {
    say "Verbose option not present in arguments";
} # OUTPUT: «Verbose option not present in arguments␤»

Since there is also a prefix version of not, this example reads better as:

my @args = <-a -e -b>;
my $verbose-selected = any(@args) eq '-v' | '-V';
if not $verbose-selected {
    say "Verbose option not present in arguments";
} # OUTPUT: «Verbose option not present in arguments␤»

From language/operators

See Original text in context

multi sub prefix:<not>(Mu $x --> Bool:D)

Evaluates its argument in Boolean context (and thus collapses Junctions), and negates the result. Please note that not is easy to misuse. See traps.