/home/alex>perl -Mstrict -w -E 'say fileno(STDERR) ? "Filehandle" : "NA"'
Filehandle

/home/alex>perl -Mstrict -w -E 'say fileno(STDOUT) ? "Filehandle" : "NA"'
Filehandle

/home/alex>perl -Mstrict -w -E 'say fileno(STDIN) ? "Filehandle" : "NA"'
NA

/home/alex>