Just to strengthen what bluto says, here's the documentation for $! from perlvar(1): 
     $!      If used numerically, yields the current value of the
             C "errno" variable, or in other words, if a system
             or library call fails, it sets this variable.  This
             means that the value of $! is meaningful only
             immediately after a failure:

                 if (open(FH, $filename)) {
                     # Here $! is meaningless.
                     ...
                 } else {
                     # ONLY here is $! meaningful.
                     ...
                     # Already here $! might be meaningless.
                 }
                 # Since here we might have either success or failure,
                 # here $! is meaningless.

             In the above meaningless stands for anything: zero,
             non-zero, "undef".  A successful system or library
             call does not set the variable to zero.

             If used an a string, yields the corresponding system
             error string.  You can assign a number to $! to set
             errno if, for instance, you want "$!" to return the
             string for error n, or you want to set the exit
             value for the die() operator.  (Mnemonic: What just
             went bang?)

             Also see "Error Indicators".

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In reply to Re^2: readline succeeds but sets $! = EBADF by sgifford
in thread readline succeeds but sets $! = EBADF by ambrus

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