This code block will do an inplace edit for the file mydata.txt. $^I allows you to specify the backup file extension.

You can turn off backup feature but it is dangerous or rather not a good idea.

code below is untested

{
  local $^I = ".bak";
  local *ARGV;
  @ARGV = "mydata.txt";
  while (<>) {
    s/idTag/newTag/g;
    print;
  }
}
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