As a rule of thumb in Unix: You are safe if you check $! (i.e. errno) only after a call actually fails. You can't determine failure from $! alone.

Unless the documentation specifically says how it handles $!, you should think of it as the last thing that failed, somewhere buried deep down in the code you called.

Update:I spoke to soon wrt docs. I noticed that the readline docs do say they set $! on an error, but they don't directly mention what they return. In an example it seems to imply that the return value is undef in that case.

In reply to Re: readline succeeds but sets $! = EBADF by bluto
in thread readline succeeds but sets $! = EBADF by ambrus

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