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 );