It almost sounds like you want something like this (warning: destructive to the array since it removes items off the beginning because they've already been compared):

my @array = ( 1, 2, 3, 4, 1, 2, 3 );
my $matches = 0;
while( @array ) {
  my $candidate = shift @array;
  for( @array ) { $matches++ if $candidate == $_ }
}
print "Got $matches matches\n";
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Update: I like toolic's approach below better as it solves the problem "does my list have more than three duplicates of any one element" without as many comparisons; original question was fuzzy so . . .