use strict;
use warnings;
my $data = "a\nb\n";

open my $fh, '<', \$data;
my $fileno = fileno $fh;
printf "fileno: %s\n", defined $fileno ? $fileno : '(undef)';

print for <$fh>;
close $fh;
$fileno = fileno $fh;

printf "fileno: %s\n", defined $fileno ? $fileno : '(undef)';
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