use strict;
use warnings;
use List::Util 'first';

my @ary = qw/EVENT_A EVENT_A EVENT_B EVENT_A EVENT_B EVENT_B/;

my($a_it, $b_it) 
    = map{ my $w = "EVENT_$_"; mk_it(\@ary, sub{ shift eq $w }) } qw/A B/;

while( defined(my $eva = $a_it->()) ) {
    my $evb = $b_it->();
    die "A/B event mismatch for $ary[$eva]" unless defined $evb && $evb>$eva;
    print "$ary[$eva]($eva) => $ary[$evb]($evb)\n";
}   

sub mk_it {
    my($pos, $aref, $wanted) = (0, @_);
    sub {
        my $rv = first{$wanted->($aref->[$_])} $pos..$#$aref;
        $pos = defined $rv ? $rv+1 : $pos+1;
        $rv;
    }
}