my %HoH = %{+shift};

Did I miss something? I don't see the word "shift" in the entire thread. Anyway, that takes the first parameter, dereferences it as a hashref and assigns the resulting hash to %HoH. The plus sign is needed for preventing Perl to take $shift as a reference and instead use the shift function. shift without arguments operates on @_ (UPDATE: or @ARGV depending on the scope).

lc is "lowercase". I'm not sure about the comma.

grep { $count!=$count{$_} } map { ++$count{$_};$_ } map { keys %{$HoH{$_}} } keys %HoH
the result of this line is stored in which variable?

That returns a list but, since it's surrounded with [ and ], the for loop gets a reference to that list. It's not stored anywhere. It's visible inside the loop as $_.

I see it the other way around, that is, s/obscure/brilliant/.

lc is "lowercase". I'm not sure about the comma.

It's there because of this:

$ cat foo.pl
my %a;
$_='fOo';

$a{lc}++;
$a{lc()}++;
$a{+lc}++;
$a{lc,}++;

$ perl -MO=Deparse foo.pl
my %a;
$_ = 'fOo';
++$a{'lc'};
++$a{lc $_};
++$a{lc $_};
++$a{lc $_};
foo.pl syntax OK
[download]

So it's there for disambiguating too, just as +shift. I chose the latter form because in this context (involving the ++ operator) it seemed to me to be the least obtrusive and visually distracting.

Sorry but some syntaxic piece of code are really obscur. i am not as comfortable as you with perl. do you mind giving me a short explanation on the elements hereunder?

sub perl_mouse {
    my %HoH = %{+shift}; # I know @_ to get the parameter but not this
+ one
}

$saw{lc,} # what is lc,?
[download]

• perldoc -f shift
• perldoc -f li

Thanks for your accurate explanation. I knew the functions shift and lc. In fact, the tricky points were the comma and the plus.

I have the habit to read the documentation but I don't see very often this kind of syntax in it. to imagine how works the array reference of a grep followed by 2 map in a loop is not easy for everybody I guess.

but thank to the monks, everybody can learn.

I see it the other way around, that is, s/obscure/brilliant/.
I would say s/obscure/brilliant_but_obuscure/ ;