This is a known issue with xtclsh in ISE 10.1 software. It will occur when xtclsh parses files that contain more than 4,150 characters on a given line.
This issue occurs only on Windows. The problem does not occur on Linux.
The issue has been fixed in ISE 11 software.
To work around this issue in ISE 10.1 software, use HDL techniques to write strings in shorter form. For example, to write a 4156-bit wide vector of zeros, use the following HDL syntax:
Init_Value => conv_std_logic_vector(0, 4156)