As a debugging aid (since you're using ps to inspect things) if your OS allows you might change $0 to add extra info.

## Prelude use'es as before

my $original_name = $0;
$0 = qq{$original_name($$): TRYING TO GET LOCK};

## Your open and flock lines . . .

$0 = qq{$original_name($$): HOLDING FLOCK};

sleep;
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