I'd guess you have a line that doesn't start with a timestamp. How about

last if $log_line =~ /^\d{4}-\d\d-\d\d \d\d:\d\d:\d\d/ && $log_line lt
+ $earliest;
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Or make your test code print out the line that made it stop:

if ($log_line lt $earliest) {
   use Data::Dumper;
   $Data::Dumper::Useqq = 1;
   print Dumper("stopped on $log_line"),"\n";
}
[download]