If my @capitals array contains only 'London' ...

An array contains only 'London' if it has only one element and that element is the string 'London':

c:\@Work\Perl\monks>perl -wMstrict -le
"my $string = 'London';
 my @capitals = split /,/, $string;
 if (@capitals == 1 && $capitals[0] eq 'London') {
   print q{contains only 'London'}
   }
"
contains only 'London'
[download]

... OR it contains all elements of my @capitals.

I don't understand what this requirement means. Can you please elaborate?