grep repetitive elements?

[Jennifer Pan]

I have two list, 
@list1 and @list2, each element @list1 is unique (could be a hash I
suppose)

I want to find out all the elements @list2 that is an element in list1
and have appeared two or more times

I want to use GREP but I don't know how to grep certain element only
when it happens >2 times, or is it possible?

foreach $L1 (@list1) {
        if ( grep /$L1/ @list2 more than twice?) {
        print $L1;
}

many thanks!!

_jennifer

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