You could test

if (@ARGV or !-t STDIN and !eof()) {
    while (<>) {
        ...
[download]

(but any non-input file arguments would need to be removed from @ARGV first, of course)

This should work with input file arguments on the command line, heredoc input (<<), input redirect (<), and piping into the program (|).

Update: actually, I think you don't even need the eof().