$ perl -e '
open(F,"/etc/passwd");
print select(F),"\n";
print select(STDOUT),"\n";
close(F);
'
main::F

$ perl -e '
open(F,"/etc/passwd");
print select((select(F),$|++)[0]),"\n";
print select(STDOUT),"\n";close(F);
'
main::F
main::STDOUT