The variables $@, $!, $^E, and $? are called as error indicators in perl. These variables also contain information about different types of error conditions that may appear during execution of a Perl program. The variables are shown ordered by the "distance" between the subsystem which reported the error and the Perl process. They corre- spond to errors detected by the Perl interpreter, C library, operating system, or an external program, respectively.

$! is set to the C library's "errno" which inturn mapped to error string in perl compiler.

For more information on this indicators refer perlvar