I would first split and then process in chunks of 21 items:

my @values = split /\s+/, $weatherdata;
croak "Invalid item count in weather data"
    unless @values % 21 == 0;
while(my @row = splice @values, 0, 21 ) {
....
}

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In reply to Re^5: Repeating a capture group pattern within a pattern by Corion
in thread Repeating a capture group pattern within a pattern by mldvx4

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