sub solve {
    my ($target, @nums) = @_;
    return unless @nums;
    if ( @nums == 1 ) {
        return unless eval($nums[0]) eq $target; # eq for epsilon fuzz
        return @nums;
    } else {
        my @sol;
        for my $i ( 0 .. $#nums - 1 ) {
            for my $k ( $i + 1 .. $#nums ) {
                my @rem = @nums;
                my $nk = splice @rem, $k, 1;
                my $ni = splice @rem, $i, 1;
                for ( combine($ni, $nk) ) {
                    unshift @rem, $_;
                    push @sol, solve($target, @rem);
                    shift @rem;
                }
            }
        }
        return @sol;
    }
}

sub combine {
    my ($x, $y) = @_;
    my ($vx, $vy) = map eval, $x, $y;
    my @combi = map operate($x, $y, $_), qw(+ *);
    push @combi, operate($x, $y, '-');
    push @combi, operate($y, $x, '-') if $vx ne $vy;
    push @combi, operate($x, $y, '/') if $vy;
    push @combi, operate($y, $x, '/') if $vx and $vx ne $vy;
    @combi;
}

sub operate {
    my ($x, $y, $op) = @_;
    precedes($op, $_) and $_ = "($_)" for $x, $y;
    $op = " $op " if precedence($op) >= 3;
    "$x$op$y"
}

sub precedes {
    my ($op, $expr) = @_;
    precedence($op) < precedence($expr);
}

sub precedence {
    for ( shift ) {
        /\-/ and return 4;
        /\+/ and return 3;
        /\// and return 2;
        /\*/ and return 1;
        return 0;
    }
}

__END__