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>;
}
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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 = <>; }
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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.

So, there is no (non-binding) way to check if STDIN contains data on win32? Is there any Win32 API functions that can come to the resuce?
If we wanted to ask for console input and move on if none supplied within x secs, is another situation where checking STDIN would be useful (..and no, alarm doesn't work in this situation on win32).

-t STDIN
always seem to return 1?!

Not for me:

C:\> perl -lwe "printf '<%s>', -t STDIN"
<1>
C:\> perl -e1 | perl -lwe "printf '<%s>', -t STDIN"
<>
C:\> perl -lwe "printf '<%s>', -t STDIN" < AUTOEXEC.BAT
<>
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- tye