Re^5: how to read STDIN interactively

The undef being returned isn't the value of $/, it's a signal that EOF was reached.

Your are mistaken about the behaviour being different when $/ eq "\n". It doesn't matter what $/ is. If you only have one line, the second read will be undef.

$ echo "abc" | perl -E'$/ = undef; say defined(<>)||0 for 1..2;'
1
0

$ echo "abc" | perl -E'$/ = "\n"; say defined(<>)||0 for 1..2;'
1
0
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You saw a difference in your tests because you passed two lines before EOF with $/ eq "\n", but only one line before EOF with !defined($/).

By the way, relying on the user to close STDIN between inputs creates a stinky interface.

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Re^6: how to read STDIN interactively by Allasso (Monk) on Feb 25, 2011 at 13:57 UTC
It looks like I am lacking some needed basic prerequisite stuff to understand what you all are talking about. I'm gonna need to do some studying. thanks much.	[reply]
Re^7: how to read STDIN interactively by ikegami (Patriarch) on Feb 25, 2011 at 19:14 UTC
Think of what it takes to stop `while (<>)` (which is short for `while (defined($_ = <>))`).	[reply] [d/l] [select]