DATA is not set until the file is fully compiled. Change
BEGIN { load(); }
to just
load();

Whatever code is at the top level of the .pm or in BEGIN blocks will get executed before use or require returns, so it's still done "at the pt. when a program 'use's my module". Also, it will only get executed the first time use or require is called in a given interpreter, so it's safe to use the module from multiple files. For example:

# mod.pm
package mod;
sub load {
   print(<DATA>);
   print("loaded\n");
}
load();
1;
__DATA__
some data
[download]

# script.pl
BEGIN { print("Before 'use mod;'\n"); }
use mod;
BEGIN { print("After 'use mod;'\n"); }
use mod;
[download]

output:

Before 'use mod;'
some data
loaded
After 'use mod;'
[download]

Updated

You're right ikegami,
Just putting a load(); in the module is the cleanest way to do the job.
Thanks!
allan

PS:
I was trying load(); in a BEGIN block in the module, -- that didn't work, and now I see why.

One reason a BEGIN block "doesn't do the job" (probably among several reasons) is described in perldata:

Note that you cannot read from the DATA filehandle in a BEGIN block: the BEGIN block is executed as soon as it is seen (during compilation), at which point the corresponding __DATA__ (or __END__) token has not yet been seen.

In SelfLoader, you can read about how to use the DATA filehandle in packages other than package main.

Without seeing your code, I think you're trying to solve the wrong problem. At load time, all bare code within a module is run and you don't even need a BEGIN block. My guess is that your __DATA__ section and your module get mixed up. The DATA filehandle belongs to the __DATA__ section of the main program. If you want to read the __DATA__ section of your module, you need to use My::Module::DATA.

to Corion ...
Yes indeed, it was an X/Y question, -- no need for BEGIN here. Thanks -- allan

Is there no way to have a module read "its own" __DATA__ section at 'use time' ?

Here's the code:
Read more... (14 kB)
allan