STDIN is always there.

Maybe you want to detect instead of whether your script is supposed to get interactive input or input piped from a file, or another program?

if( -t and !@ARGV) {
    die "Need a file or stream on standard input";
};

while (<>) {
    ...
};
[download]

>perl -w tmp.pl
Need a file or stream on standard input at tmp.pl line 2.

>perl -w tmp.pl tmp.pl

>perl -w tmp.pl <tmp.pl
[download]