6.5.1. $^T and time( )

For example, Perl uses $^T with the -M, -C, or -A file test operators. As a result, files created after the child server's startup are reported as having a negative age when using those operators. -M returns the age of the script file relative to the value of the $^Tspecial variable.

If you want to have -M report the file's age relative to the current request, reset $^T, just as in any other Perl script. Add the following line at the beginning of your scripts:

local $^T = time;

You can also do:

local $^T = $r->request_time;

If this correction needs to be applied to a lot of handlers, a more scalable solution is to specify a fixup handler, which will be executed during the fixup stage:

sub Apache::PerlBaseTime::handler {
    $^T = shift->request_time;
    return Apache::Constants::DECLINED;
}

and then add the following line to httpd.conf:

PerlFixupHandler Apache::PerlBaseTime

Now no modifications to the content-handler code and scripts need to be performed.

6.5.2. Command-Line Switches

Most command-line switches have special Perl variable equivalents that allow them to be set/unset in code. Consult the perlvar manpage for more details.

mod_perl provides its own equivalents to -w and -T in the form of configuration directives, as we'll discuss presently.

Finally, if you still need to set additional Perl startup flags, such as -d and -D, you can use the PERL5OPT environment variable. Switches in this variable are treated as if they were on every Perl command line. According to the perlrun manpage, only the -[DIMUdmw] switches are allowed.

6.5.3. Compiled Regular Expressions

my $pattern = '^\d+$'; # likely to be input from an HTML form field
foreach (@list) {
    print if /$pattern/o;
}

This is usually a big win in loops over lists, or when using the grep( ) or map( ) operators.

In long-lived mod_perl scripts and handlers, however, the variable may change with each invocation. In that case, this memorization can pose a problem. The first request processed by a fresh mod_perl child process will compile the regex and perform the search correctly. However, all subsequent requests running the same code in the same process will use the memorized pattern and not the fresh one supplied by users. The code will appear to be broken.

Imagine that you run a search engine service, and one person enters a search keyword of her choice and finds what she's looking for. Then another person who happens to be served by the same process searches for a different keyword, but unexpectedly receives the same search results as the previous person.

There are two solutions to this problem.

The original code fragment would be rewritten as:

my $pattern = '^\d+$';
eval q{
    foreach (@list) {
        print if /$pattern/o;
    }
}

If we were to write this:

foreach (@list) {
    eval q{ print if /$pattern/o; };
}

the regex would be compiled for every element in the list, instead of just once for the entire loop over the list (and the /o modifier would essentially be useless).

However, watch out for using strings coming from an untrusted origin inside eval—they might contain Perl code dangerous to your system, so make sure to sanity-check them first.

This approach can be used if there is more than one pattern-match operator in a given section of code. If the section contains only one regex operator (be it m// or s///), you can rely on the property of the null pattern, which reuses the last pattern seen. This leads to the second solution, which also eliminates the use of eval.

The above code fragment becomes:

my $pattern = '^\d+$';
"0" =~ /$pattern/; # dummy match that must not fail!
foreach (@list) {
    print if //;
}

The only caveat is that the dummy match that boots the regular expression engine mustsucceed—otherwise the pattern will not be cached, and the // will match everything. If you can't count on fixed text to ensure the match succeeds, you have two options.

If you can guarantee that the pattern variable contains no metacharacters (such as *, +, ^, $, \d, etc.), you can use the dummy match of the pattern itself:

$pattern =~ /\Q$pattern\E/; # guaranteed if no metacharacters present

The \Q modifier ensures that any special regex characters will be escaped.

If there is a possibility that the pattern contains metacharacters, you should match the pattern itself, or the nonsearchable \377 character, as follows:

"\377" =~ /$pattern|^\377$/; # guaranteed if metacharacters present