Like jimpudar I got the OP code to work with small changes. I am just a little worried that in jimpudar approach we are not checking for the possibility that @{ $ts{$timestamp} } might become empty which the code below checks:

while ( <FA> ) {
    ...
    while ( $tot-- and defined(my $id = shift @{ $ts{$timestamp} }) ){
        print join(",",$life, $id, $cls )."\n";
    }
}
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