what about this:

# sample related values
my %val  = ('14:00:00' => '200',
            '15:00:00' => '100',
            '16:00:00' => '300');

my @time = ('15:00:00','14:00:00','15:00:00','16:00:00');

my %hash;

for (@time) {
  $hash{$_} += $val{$_};
}
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Or am I misunderstanding the question?

cLive ;-)