Anno is quite right. Here's a quick explanation.

When you call a subroutine, the arguments are flattened into a list.

sub foo {
   print("There are ", 0+@_, " args\n");
   print("They are @_\n");
}

my @a = qw( a b c );
my $n = 123;

foo(@a, $n);
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There are 4 args
They are a b c 123
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For this reason, an array reference is usually passed instead of the contents of the array.

foo(\@a, $n);
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There are 2 args
They are ARRAY(0x1829aec) 123
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In your case, it would be

populateEventManifest(\@projectEvents, $eventManFh);  # <-- extra \

sub populateEventManifest {
    my( $events, $eventManFh ) = @_;                  # <-- scalar
    foreach my $event (@$events) {                    # <-- extra $
        $event->print($eventManFh);
    }
}
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Anno provided an alternative solution.