Codility task Equi, solution in perl with score 100 and O(n), see http://codility.com/demo/take-sample-test/

codility_equi.pl

use strict;
use warnings;
use v5.10;
use List::Util qw(sum first);
 
sub equi {
  my (@A)   = @_;
  my $right = sum(@A);
  my $size  = scalar @A;
 
  my $p     = first {
    state $pivot  = 0;
    state $left  += $pivot;
    $pivot  = $A[$_];
    $right -= $pivot;
    
    $left == $right
  } 0 .. $size - 1;
 
  $p // -1
}
 
use Test::More tests => 4;
 
is(equi(-7,1,5,2,-4,3,0),3,"example");
is(equi(1..5),-1,"simple");
is(equi(),-1,"trivial");
is(equi(555),0,"single");