How long are your lines? How many columns are there on average? Is the data all real numbers? If so, to what precision and with what range of values? If not, what else is there? How much memory do you have to spare? Why do you want to keep it in memory? How will you be accessing it?

My immediate reaction is to say anything other than a simple array of strings (lines) is likely to take many gigabytes.

Size of scalars varies depending on contents and your perl configuration. In general, references or integers (that have never been used in string context) will take the least amount of memory. Floating point numbers take more. Strings take more than that. Mixed strings/numbers (including numbers that have been used in a string context, so have the string representation cached) even more.