Perl Cd Bookshelf [Electronic resources] نسخه متنی

12.2.1. Problem

You usually want a program to fail if it tries to load a module that
is missing or doesn't compile. Sometimes, though, you'd like to
recover from that error, perhaps trying an alternative module
instead. As with any other exception, you insulate yourself from
compilation errors with an eval.

You don't want to use eval {
BLOCK }, because this traps
only runtime exceptions, and use is a compile-time
event. Instead, you must use eval
"string" to catch compile-time problems as well.
Remember, require on a bareword has a slightly
different meaning than require on a variable. It
adds a ".pm" and translates double-colons into
your operating system's path separators, canonically
/ (as in URLs), but sometimes
\, :, or even
. on some systems.

If you need to try several modules in succession, stopping at the
first one that works, you could do something like this:

BEGIN {
my($found, @DBs, $mod);
$found = 0;
@DBs = qw(Giant::Eenie Giant::Meanie Mouse::Mynie Moe);
for $mod (@DBs) {
if (eval "require $mod") {
$mod->import( ); # if needed
$found = 1;
last;
}
}
die "None of @DBs loaded" unless $found;
}

We wrap the eval in a BEGIN block to ensure the
module-loading happens at compile time instead of runtime.