See perlvar#%! and/or Errno

$ perl -le " print $_, ' ', $!=$_  for 1 .. 1555" |ack child
10 No child processes
128 There are no child processes to wait for.
1406 Cannot create a top-level child window.
1442 The window is not a child window.

$ perl -MErrno -le " printf qq/%5s %s\n/ , Errno->$_ , $_ for grep /CH
+ILD/, keys %! "
 1020 ERROR_KEY_HAS_CHILDREN
 8332 ERROR_DS_CHILDREN_EXIST
 1442 ERROR_NOT_CHILD_WINDOW
 8422 ERROR_DS_CANT_RETRIEVE_CHILD
 1406 ERROR_TLW_WITH_WSCHILD
 1436 ERROR_CHILD_WINDOW_MENU
  129 ERROR_CHILD_NOT_COMPLETE
 1021 ERROR_CHILD_MUST_BE_VOLATILE
   10 ECHILD
 1445 ERROR_NON_MDICHILD_WINDOW
  128 ERROR_WAIT_NO_CHILDREN
 8613 ERROR_DS_EXISTING_AD_CHILD_NC

$ perldoc -v %!
    %OS_ERROR
    %ERRNO
    %!      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 rece
+nt
            error was "No such file or directory" (or its moral equiva
+lent:
            not all operating systems give that exact error, and certa
+inly
            not all languages). To check if a particular key is meanin
+gful
            on your system, use "exists $!{the_key}"; for a list of le
+gal
            keys, use "keys %!". See Errno for more information, and a
+lso
            see "$!".

            This variable was added in Perl 5.005.
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