When referenced,$! retrieves the current value of the Cerrno integer variable. If$! is assigned a numerical value, that value is stored inerrno. When referenced as a string,$! yields the system error string corresponding toerrno.
Many system or library calls seterrno if they fail, to indicate the cause of failure. They usually donot seterrno to zero if they succeed. This meanserrno, hence$!, is meaningful onlyimmediately after afailure:
if (open my $fh, "<", $filename) {# Here $! is meaningless.... } else {# ONLY here is $! meaningful....# Already here $! might be meaningless. } # Since here we might have either success or failure, # $! is meaningless.Here,meaningless means that$! may be unrelated to the outcome of theopen() operator. Assignment to$! is similarly ephemeral. It can be used immediately before invoking thedie() operator, to set the exit value, or to inspect the system error string corresponding to errorn, or to restore$! to a meaningful state.
Mnemonic: What just went bang?
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