When you read in your first line, it has the form      1\n with a trailing newline. So when you say:      $_ =~ s/$_/$_ 1.0/g; you are saying, "If $_ can be found in the variable $_ (which of course it can), substitute for the entire value, that same value with " 1.0" on the end. Which results in the string:      1\n 1.0 Which is just what you report.

Doing a substitution on the whole string is a bit odd anyway. And you don't need the /g. How about (staying with the level of verbose clarity you are using):

foreach $txtfile (glob("*.TXT")) {
   print $txtfile;
   print "\n";
   open FILE, "$txtfile";
   open NEWFILE, ">$txtfile.tmp";
   while (<FILE>){
      chomp $_;
      if ($. == 1){
         $_ .= " 1.0";     # i.e. $_ = $_ . " 1.0";
      }
      print NEWFILE $_, "\n";
   }
   close FILE;
   close NEWFILE;
}
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Update: A more streamlined version might be:

foreach $txtfile (glob("*.TXT")) {
   print $txtfile;
   print "\n";
   open FILE, "$txtfile";
   open NEWFILE, ">$txtfile.tmp";
   while (<FILE>){
      $_ =~ s/\n/ 1.0\n/ if $. = 1;
      print NEWFILE $_;
   }
   close FILE;
   close NEWFILE;
}
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