The built-in method .first fulfills the requirements of this task.
It takes any smart-matcher as a predicate. The :k adverb makes it return the key (i.e. numerical index) instead of the value of the element.
my @cities =
{ name => 'Lagos', population => 21.0 },
{ name => 'Cairo', population => 15.2 },
{ name => 'Kinshasa-Brazzaville', population => 11.3 },
{ name => 'Greater Johannesburg', population => 7.55 },
{ name => 'Mogadishu', population => 5.85 },
{ name => 'Khartoum-Omdurman', population => 4.98 },
{ name => 'Dar Es Salaam', population => 4.7 },
{ name => 'Alexandria', population => 4.58 },
{ name => 'Abidjan', population => 4.4 },
{ name => 'Casablanca', population => 3.98 },
;
say @cities.first(*<name> eq 'Dar Es Salaam', :k);
say @cities.first(*<population> < 5).<name>;
say @cities.first(*<name>.match: /^A/).<population>;