while (
   /
      ^
      (\d{4}-\d\d-\d\d)
      .*dates processed:[ ]
      ( (?:\d{4}-\d\d-\d\d,[ ])* \d{4}-\d\d-\d\d )
      $
   /mg
) {
   my $on        = $1;
   my $procesed  = $2;
   my @processed = split(/, /, $processed);

   # Do something with $on and @processed.
}

##</code><code>##

while (<$fh>) {
   my ($on, $processed) = /
      ^
      (\d{4}-\d\d-\d\d)
      .*dates processed:[ ]
      ( (?:\d{4}-\d\d-\d\d,[ ])* \d{4}-\d\d-\d\d )
      $
   /
      or next;

   my @processed = split(/, /, $processed);

   # Do something with $on and @processed.
}