Perl: do not leak FD to child process. This fixes #5797.

Centreon Connector Perl works parsing Perl scripts in a main
process. When a script execution is requested, the Connector
fork()s itself and child processes run one of the already parsed
script. However because of the fork() mechanism, children inherit
file descriptors of pipes already open towards previously forked
children. This behavior can freeze Centreon Connector Perl
(read/write on the other end of pipe maintained alive by a dead
process). This commit closes all unnecessary file descriptors in
children.