"My another question is about the reason of using the ++. If the 0(false) is added to 1, what's the incrementing for? It's not like an iterator's or a pointer's moving we normally see. The increasing didn't change anything seemingly."

I don't understand what point you're making here. Take a look at these two scripts: the first with ++, the second without.

$ perl -Mstrict -Mwarnings -le '
    my @fields = qw{a a b c b a};

    my %seen;

    for my $field (@fields) {
        print $field, $seen{$field}++ ? "PUSH" : "SET";
    }
'
aSET
aPUSH
bSET
cSET
bPUSH
aPUSH
[download]

$ perl -Mstrict -Mwarnings -le '
    my @fields = qw{a a b c b a};

    my %seen;

    for my $field (@fields) {
        print $field, $seen{$field} ? "PUSH" : "SET";
    }
'
aSET
aSET
bSET
cSET
bSET
aSET
[download]

That may clarify whatever is causing confusion or misunderstanding. If not, please supply a similarly short piece of code to explain your point.

"... I decided to find some artical about Perl Ops priority to read first."

Take a look at "perlop - Perl operators and precedence". There's a table of precedence and associativity at the start of the documentation. While you're there, you might as well also look at these sections further down the page: "Auto-increment and Auto-decrement" for "++" and Conditional Operator for "?:".

Thanks. My head must had been frozened. Your example is very helpful and interested. $seen only make a tag to what have seen in the @_, for some fields might be repeated in the @_.