I am working on a spreadsheet that compiles data from other spreadsheets. I
refer to data in the other spreadsheets with a formula like the one below:

=[filename.xls]Sheet1!$B$1

I would like to have a cell where I could change the filename, and have all
of cells that use that filename change. In other words, I would like to have
a variable in the formulas that refers to the file, like this:

=["variable".xls]Sheet1!$B$1

So I could specify that the variable is filename1, run my calculations, then
specify that the variable is filename2 and run the next set of calculations,
and so on.

Is this possible? If so, how do you do it?

Thanks for your help!