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
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).