From type/Supply

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method on-close(Supply:D: &on-close --> Supply:D)

Returns a new Supply which will run &on-close whenever a Tap of that Supply is closed. This includes if further operations are chained on to the Supply. (for example, $supply.on-close(&on-close).map(*.uc)). When using a react or supply block, using the CLOSE phaser is usually a better choice.

my $s = Supplier.new;
my $tap = $s.Supply.on-close({ say "Tap closed" }).tap(
    -> $v { say "the value is $v" },
    done    => { say "Supply is done" },
    quit    => -> $ex { say "Supply finished with error $ex" },
);
 
$s.emit('Raku');
$tap.close;        # OUTPUT: «Tap closed␤»