Let's put the data of the CalendarItem you've shown in its own variable:

my $hr_CalendarItem = $response_xml->{'soap:Body'}->{'m:FindItemRespon
+se'}->{'m:ResponseMessages'}->{'m:FindItemResponseMessage'}->{'m:Root
+Folder'}->{'t:Items'}->{'t:CalendarItem'};
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Now the subject and wanted value can be found thus:

my $subject = $hr_CalendarItem->{'t:Subject'};
my $value = $hr_CalendarItem->{'t:ExtendedProperty'}->{'t:Value'};
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