use strict;
use warnings;
use feature 'say';

# Sample data
my @events = (
    'EVENT A',
    'EVENT A',
    'EVENT B',
    'EVENT A',
    'EVENT A',
    'EVENT A',
    'EVENT B',
    'EVENT A',
    'EVENT B'
);

my ($first_A, $first_B);
my $state = 'find_A';
my %parse = (
    find_A => sub {
        my ($event, $index) = @_;
        if ($event eq 'EVENT A') {
            $first_A = $index;
            $state = 'find_B';
        }
    },
    find_B => sub {
        my ($event, $index) = @_;
        if ($event eq 'EVENT B') {
            $first_B = $index;
            $state = 'finished';
        }
    }
);

while (my ($index, $event) = each @events and $state ne 'finished') {
    $parse{$state}->($event, $index);
}

say 'First occurrence of A: ', $first_A // 'none';
say 'First occurrence of B: ', $first_B // 'none';