Look into truth tables, and test -t STDIN first

$ perl -le " warn 0 + @ARGV; warn 0 + -t STDIN"
0 at -e line 1.
1 at -e line 1.

$ perl -le " warn 0 + @ARGV; warn 0 + -t STDIN"  a b c d e f
6 at -e line 1.
1 at -e line 1.

$ perl -le " warn 0 + @ARGV; warn 0 + -t STDIN"  a b c d  < NUL
4 at -e line 1.
0 at -e line 1.

$ perl -le " warn 0 + @ARGV; warn 0 + -t STDIN"   < NUL
0 at -e line 1.
0 at -e line 1.
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