If you are passing in command line args AND are redirecting/piping STDIN into your perl script, that you must not use <> to loop over the STDIN. Instead, use <STDIN> and refer to your arguments via the @ARGV array. Otherwise, Perl thinks that $ARGV[0] is a file name when it encounters just a <>.

This works:

% prepend.pl bob < some.txt

while <STDIN> {
  print "$ARGV[0]: $_"
}
[download]

This doesn't:

% prepend.pl bob < some.txt

while <> {
  print "$ARGV[0]: $_"
}
[download]

You may also want to check out Multiple STDIN sources.