If you mean "the three consecutive values that when summed make up the highest total", then that gives an implementation strategy:

my $where = 0;
my $sum = $num[0] + $num[1] + $num[2];
for ( 1 .. $#sum-2 ) {
  my $try = $num[$_] + $num[$_+1] + $num[$_+2];
  if ( $try > $sum ) {
    $try = $sum;
    $where = $_;
  }
}
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