sub changeme{  
  local $^I = ".$bak";
  @ARGV = "somefile";
  while (<>) {
    s/foo/bar/i;
    print;
  }
}

##</code><code>##

sub changeme{
  $myvar = somesub();
  local $^I = ".$bak";
  @ARGV = "somefile";
  while (<>) {
    s/foo/$myvar/i;
    print;
  }
}