use strict; use warnings;
my $x;
$x = 'bar';
BEGIN { print "\$x in first BEGIN block: '$x'\n" }
print "runtime: \$x = '$x'\n";
BEGIN { 
    $x = 'foo';
    print "\$x in second BEGIN block: '$x'\n";
}
print "done.\n";
__END__
Use of uninitialized value $x in concatenation (.) or string at foo.pl line 4.
$x in first BEGIN block: ''
$x in second BEGIN block: 'foo'
runtime: $x = 'bar'
done.