$ perl -Mwarnings -Mstrict -le'
my @array = qw( 1 3 3 3 5 5 5 5 );
my %count;
my @new_array = map {
    ++$count{ $array[ $_ ] };
    no warnings q/uninitialized/;
    $array[ $_ ] eq $array[ $_ + 1 ] ? () : $count{ $array[ $_ ] }
    } 0 .. $#array;
print "@new_array";
'
1 3 4