if(  ! @ARGV  ) {
    # CGI code here
    my $pid = CORE::fork();
    die "Can't fork: $!\n"
        if  ! defined $pid;
    exit     # or return, or whatever makes most sense to finish CGI part
        if  $pid;
    CORE::exec( 'perl', $0, 'daemon' );
    die "Can't exec perl: $!\n";
}
# Rest of code here