You might be interested in the int2LATIN function from the Number::Latin module.
Alternatively, since you are using Win32::OLE to control Excel directly, there are methods for specifying a cell by using integers to specify a row and column. I don't remember the details and there are some situations where that method isn't available. That's why I consider Number::Latin to be indispensable when using Win32::OLE to control Excel.
Using your example of column number 703 (aka column AAA):
C:\>perl -E"use Number::Latin; say int2LATIN(703);"
C:\>perl -E"use Number::Latin; say latin2int('AAA');"