I'm going to take your title as a question, irrespective of any interpretation of what you really want to achieve. Can you instert extra text, say, from a text file, into the perl input via STDIN? Well, yes you can.

If you set @ARGV to the paths of a number of files, while(<>) will read the files instead of STDIN. But add "-" (that's a minus sign) as a file namen, and perl will also read from normal STDIN.

In summary: the following snippet will read a saved text file and input from STDIN.

@ARGV = ('grocery_list.txt', '-');
while(<>) {
    print " * $_";
}
[download]

This will read from STDIN until you somehow terminate it: read from a finite pipe, or enter ctrl-D or ctrl-Z interactively, depending on the platform.