sub func {
    my ($thing) = @_;
    $thing = defined $thing ? $thing : 'non-thing';

    # or

    if (defined $thing){
        $thing = $thing;
    }
    else {
        $thing = 'non-thing';
    }
}

##</code><code>##

sub func {
    my ($thing) = @_;
    $thing //= 'non-thing';
}