Is this what you want?

foreach $i (@n) {
  my ( $first_sixteen ) = $i =~ /^\s*(\d{16})/;
  my ( $pre, $post ) =
    $i =~ /(.{30})Some search string(.{30})/;

  warn "something not right with $i\n", next
    if grep !defined $_, $first_sixteen, $pre, $post;

  # do something with $first_sixteen, $pre, $post
}
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