Each element of %! has a true value only if $! is set to that value. For example, $!{ENOENT} is true if and only if the current value of $! is ENOENT; that is, if the most recent error was "No such file or directory" (or its moral equivalent: not all operating systems give that exact error, and certainly not all languages). The specific true value is not guaranteed, but in the past has generally been the numeric value of $!. To check if a particular key is meaningful on your system, use exists $!{the_key}; for a list of legal keys, use keys %!. See Errno for more information, and also see "$!".
This variable was added in Perl 5.005.