You need to enclose $dir/$new_date in quotes. Otherwise, Perl assumes that you are trying to perform division with two scalars.

opendir (DIR, "$dir/$date_cfg") or die "Can't read from $dir/date_cfg:
+ $!\n";
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In fact, ordinarily you would have spotted this problem right away. Perl will generate an error similar to the following:

Argument "new_date" isn't numeric for division at somescript.cgi line 
+15.
Illegal division by zero at somescript.cgi line 15.
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The "isn't numeric" warning occurs if you use the -w switch, but does not stop the program. However, since Perl will do string/numeric conversions on the fly, it will usually result in a division by zero error, which does kill the program. Only because you are using numeric data in your second argument are you not getting an error.

Cheers,
Ovid