Regexps should be tested and action taken when they fail, e.g.

DATA: while (<DATA>) {
   chomp;
   $_ or next DATA; # allow empty lines
   /(\d\d):(\d\d):(\d\d)/ or die "corrupted data at line $.: $_\n"; # 
+but no other deviation
   my $hours = $1; 
   etc.
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