Ah, ok. Try these changes:

# no need for this loop:
# for my $w_data (@$weather_data) {
#     push @data, $w_data;    
# }
 
# Now add 'report_date' key and its value at the appropriate level.
#push @data, $weather_data->[0]{'xml_api_reply'}{'weather'}{'report_da
+te'} = { '-data' => $today 
#};

push @{$weather_data->[0]{'xml_api_reply'}{'weather'}{'report_date'}},
+ { '-data' => $today };

# print Dumper(\@data);
print Dumper(\$weather_data);
[download]