with role

my role CachedRoutine[$orig] {
    has %!cache;
    method postcircumfix:<( )>($c) {
        my $key := $c.gist;
        %!cache.exists($key) ??
            %!cache{$key} !!
            (%!cache{$key} = $orig(|$c))
    }
}

sub trait_mod:<is>(Routine $r, :$cached!) {
    $r does CachedRoutine[$r.clone()]
}

sub foo($a) is cached {
    say "I was called with $a";
    return 2 * $a;
}

say foo(1);
say foo(2);
say foo(1);
say foo(2);

with wrap

sub trait_mod:<is>(Routine $r, :$cached!) {
    my %cache;
    $r.wrap(-> |$c {
        my $key := $c.gist;
        %cache.exists($key) ??
            %cache{$key} !!
            (%cache{$key} = callsame)
    })
}

sub foo($a) is cached {
    say "I was called with $a";
    return 2 * $a;
}

say foo(1);
say foo(2);
say foo(1);
say foo(2);