There are two probelms with your open statement: it should be opendir and you are missing a path separator:

foreach my $client (@client_directories)
{
    opendir(C_DIR, "$base_directory/$client") or die "Could not open c
+lient directory $base_directory/$client: $!\n";
    my @config_directories = grep { $_ ne '.' and $_ ne '..' } readdir
+ C_DIR;
    close C_DIR;
    print "@config_directories\n";
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