sub foo {
    my $arg = shift @_;
    state %cache;

    return $cache{$arg} if exists $cache{$arg};

    return $cache{$arg} = something_costly( $arg );
}
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Test code:

use Immutable::UntestableCode qw( foo );

my $real_thing = \&Immutable::UntestableCode::something_costly;
my $calls = 0;
*Immutable::UntestableCode::something_costly = sub {
    $calls++;
    return $real_thing->( @_ );
};

foo( 1 );
is( 1, $calls );
foo( 1 );
is( 1, $calls );
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