It really is not possible to overwrite data in file. You must replace the file with edited version of itself. (This would be true of any language) The perl runtime flags /p and /i should be a big help in doing this.

Process the file one line at a time.

• If the current line is a 'Parameters' line, extract the vector into a global array.
• Else if it is a 'Name' line, extract the block number into a global scalar.
• Else if it is a 'LowerLimit' line, update the value. (use $vector[$block])
• Else if it is a 'UpperLimit' line, update the value.
• Else do nothing

If you still have problems, post explicit questions.