Erm, no it does either arguments or STDIN if none were given just like the documentation in perlop and the code you copied there from says. Read it again:

unshift( @ARGV, '-' ) unless @ARGV;
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Put a string '-' (which is shorthand to open to read from STDIN) onto the front of @ARGV unless there are already arguments in @ARGV.

If you're getting the behavior you're seeing out of your test1.pl then something very strange is going on.

$ cat my_test1.pl                         
#!/usr/bin/perl

use strict;
use warnings;

while (<>){
  print;
}
$ cat infile                              
2 from infile
3 from infile
$ echo "testing" | perl my_test1.pl infile
2 from infile
3 from infile
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