The Perl syntax error message from the last eval() operator. If null, the last eval() parsed and executed correctly (although the operations you invoked may have failed in the normal fashion). (Mnemonic: Where was the syntax error "at"?)
Warning messages are not collected in this variable. You can, however, set up a routine to process warnings by setting $SIG{__WARN__} as described below.
Also see "Error Indicators".