use Cwd;
use constant LIMIT => 4 * 1024 * 1024 * 1000;

my @dirs;  # perhaps found with File::Find
for my $dir (@dirs) {
    my $avail = LIMIT - dir_size($dir);
    print "'$dir' has $avail free\n";
}    

sub dir_size {
    my ($dir) = @_;
    my $orig_dir = getcwd;
    chdir '$dir' or die "Unable to chdir to '$dir': $!";
    open(my $dh, '.') or die "Unable to open a dir handle for '$dir': $!";
    my $size;
    while (<$dh>) {
        next if ! -f $_;
        $size += -s $_;  # could use _ shortcut
    }
    chdir $orig_dir or die "Unable to restore dir to '$orig_dir': $!";
    return $size;
}