my ($x, $y);
if (fn(\$x, \$y)) {
    die "fn() failed\n";
}

# do stuff with $x and $y...
exit 0;

sub fn
{
    my ($fn_x, $fn_y) = @_;
    ($fn_x, $fn_y) = ('x', 'y');

    my $rc = 0;
    $rc = 1 if $fn_x ne 'x'; # never happens, just an example

    return 0;
}