It's easier to the opposite: Use STDIN if there are no args. It's better because text can always be added to STDIN, but args cannot be added once a program has started.

my $text;
if (@ARGV) {
   $text = shift(@ARGV);
} else {
   local $/;
   $text = <STDIN>;
}
[download]

If the argument is a file name whose content you want to place on the clipboard, the ARGV file handle (the default for <>) does just that.

my $text; { local $/; $text = <>; }
[download]

If you still wanted to head down your original path, your best bet would be to check whether your process's STDIN has been redirected using -t STDIN (tested). select and therefore IO::Select only work on sockets in Windows.