Since  $_ =~ s/// is exactly equivalent to s/// you can do

foreach ( @subtitle )
{
    s/º/s/;
    s/þ/t/;
    s/ª/S/;

}
[download]

I don't know what you mean in the rest of your post, though, I assume you mean something like:

s/º/s/, s/þ/t/, s/ª/S/ for @subtitle;
[download]

note: all my code is untested.