Searching twice is expensive and unnecessary:

$a="THE FOOD IS DELICIOUS";;
$p=0; @p=(); 

push @p, $p-1 while $p = 1+index $a, 'O', $p; 

print @p;;
5 6 18
[download]

Or:

$a="THE FOOD IS DELICIOUS";;
@p=(); 
push @p, $-[0] while $a =~ m[O]g;;

print @p;;
5 6 18
[download]