Remember, there's a lexical scope around the file, so

package Blubb;

...

package MAIN;
my $b = Blubb->new;

END {
  warn "END\n";
}
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is the same as

{
  package Blubb;

  ...

  package MAIN;
  my $b = Blubb->new;

  END {
    warn "END\n";
  }
}
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Since nothing captures $b, it ceases to exists as soon as the end of the lexical scope (the end of the file) is reached. If a sub were to capture $b, it would keep the variable alive until the sub was freed. This is usually during global destruction.