it's a hash-element of %!

see perlvar

       %!      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 r
+ecent
               error was "No such file or directory" (or its moral equ
+ivalent:
               not all operating systems give that exact error, and ce
+rtainly
               not all languages).  To check if a particular key is me
+aningful
               on your system, use "exists $!{the_key}"; for a list of
+ legal
               keys, use "keys %!".  See Errno for more information, a
+nd also
               see above for the validity of $!.
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