•Re^5: What can we assume in a BEGIN block ?

It's really rather obvious when you pay attention to the documented compile-time vs run-time components of things.

For example, require is executed at run-time, first compiling the referenced code, then running it immediately after (because it is essentially a super-eval, which compiles and runs code at runtime). BEGIN-blocks move the enclosed code from runtime to compiletime. Since "use" is a combination of BEGIN and require, it causes an external file to be compiled, then executed, during the compilation of the current file.

Also, variables are declared at compile time, subroutines are defined at compile time, but the initialization of variables doesn't happen until runtime.

Using basic rules like this, and paying attention to the other notes (like how //o and eval work, for example), it's always possible to work out the execution order "from first principles".

-- Randal L. Schwartz, Perl hacker
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Comment on •Re^5: What can we assume in a BEGIN block ?

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Re^6: What can we assume in a BEGIN block ?
by leriksen (Curate) on Oct 05, 2004 at 00:05 UTC

Discussion below uses this code

our $initialised = 0;
BEGIN {
    _initialise();
    $initialised = 1;
}
[download]

Initially (when I didnt understand the problem) I thought the bare (not in a BEGIN) our $initialised = 0; meant 'found a scalar declaration, allocate memory, create an entry in the symbol table that references that location,and put a 0 in that memory location. Now parse and run the BEGIN block, which will result in $initialised having a 1 in it'.

Now I (think/know) it means 'found a scalar declaration, allocate memory, create an entry in the symbol table that references that location,and generate some opcodes that will result in a 0 being put in that location at runtime.Now parse and run the BEGIN block, which will result in $initialised having a 1 in it, which will get clobbered back to 0 by the opcodes at runtime'

Thanx merlyn for taking the time to break this down for me. I hope I have it straight now.

use brain;

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