Variables are initialised when the respective code executes, and BEGIN blocks execute as early as possible, i.e. as soon as they are defined.  (And use implies BEGIN.)

#!/usr/bin/perl

use strict;
use warnings;

BEGIN {
    print "3: foo=$::foo\n";
    our $foo = 42;
    print "4: foo=$foo\n";
    BEGIN {
        print "1: foo=$foo\n";
        $foo = 43;
        print "2: foo=$foo\n";
    }
    print "5: foo=$foo\n";
    $foo = 44;
}
BEGIN {
    print "6: foo=$::foo\n";
}

__END__
Use of uninitialized value in concatenation (.) or string at ./809940.
+pl line 11.
1: foo=
2: foo=43
3: foo=43
4: foo=42
5: foo=42
6: foo=44
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Note that although our $foo = 42; has already been "seen" during compilation, the variable is still undefined in the nested BEGIN block (which runs before the outer one).