I guess by variable you mean a database field or File Column
First of all you need to get file name for the data reader
Reader parameter transformation function will do it for you
Than you need to get rid of '.csv' and 'str' parts of file name
Use EnsureNoPrefix and EnsurenoSuffix transformation functions
If you are loading the data into date field you should use Data format function
If you are a complete beginner watch out online tutorials first
Peter