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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 as 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".