This works better for the given elements. It gives the result as you need. Refer splice to know about it.

my @array = (2 ,4, 3, 8, 9, 12, 13, 20, 18, 7 );

my $percent = (scalar @array * 0.20);
my (@resultA, @resultB);

my @ordered = sort {$a <=> $b} @array;

@resultB = splice(@ordered, 0, $percent);
@resultA = splice(@ordered, (scalar @ordered - $percent));
print "@array", "\n";

for (my $i=0; $i < scalar @array; $i++)  {
    if ( grep { $array[$i] eq $_ } @resultA ) {
        $array[$i] = 'A';
    }
    elsif ( grep { $array[$i] eq $_ } @resultB ) {
        $array[$i] = 'B';
    }
    else  {
        $array[$i] = '-';
    }
}

print "@array", "\n";

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