use 5.014;

my $s = "a (d (b) (e (f) ) ) c";
say remove_par($s);

sub remove_par {
    my ( $result, $open );
    foreach my $ch ( split // => shift ) {        
        $open++ if $ch eq '(';
        $open-- if $ch eq ')';
        $result .= $ch if !$open && $ch ne ')';
    }
    die "non-matching parentheses" if $open;
    return $result;
};