# real file
perl -Mstrict -wE "open my $fh,'<','out.log' or die;say fileno($fh) ? 'FileHandle! '.fileno($fh):'NA'"
FileHandle! 3

# memory file
perl -Mstrict -wE "open my $fh,'<',\my $mem or die;say fileno($fh) ? 'FileHandle! '.fileno($fh):'NA'"
FileHandle! -1

# not existing file, closed filehandles, aliens things..
perl -Mstrict -wE "open my $fh,'<','out.logXX';say fileno($fh) ? 'FileHandle! '.fileno($fh):'NA'"
NA