++ , --
配合只希计数，判断之前已出现过的元素：
#! /usr/bin/perl
my @people = qw (able Fred Fred barney bamm-bamm wilma dino barney betty pebbles );
my %seen;
foreach (@people) {
print "I've seen you somewhere before, $_!\n"
if $seen{$_}++;
}
因为barney出现了再次，所给输出为：
I've seen you somewhere before, barney!
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