$ perl -E '
use strict;
use warnings;
say "Real \\*STDIN fileno: ", fileno(\*STDIN);
say "Real \\*DATA  fileno: ", fileno(\*DATA);
*STDIN = *DATA;
say "Fake \\*STDIN fileno: ", fileno(\*STDIN);
__DATA__
some data
'
Real \*STDIN fileno: 0
Real \*DATA  fileno: 3
Fake \*STDIN fileno: 3