That's great, I just corrected I little thing here:

my @tides = ( { time_in_seconds => 0,
                highlow       => 'Unknown', # it was high_low
                time          => 'Unknown',
               } );
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But thanks for your help. I have to do more work on that to check the dates for the next available tide and avoid printing 'Unknown'. This XML has the dates for the whole year, but this pointed me in the right direction for sure!