It's also handy to limit the scope of the block internals, such as a closure:

Checker:
{
  my %seen;

  sub Checker
  {
    my $value = shift;
    if (defined($seen{$value}))
    {
      return $value;
    }
    else
    {
      $seen{$value} = 1;
      return;
    }
  }
}
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This could also be used as a bookmark for editors, assuming care was used in choosing unique labels.