while ($line = <>) {
  # redo comes HERE
  # do something
  my $next = <>;
  if ($next !~ /^#/) {
    # if the next line DOES NOT start with a "#"
    # reuse it and restart this iteration of the loop
    $line = $next;
    redo;
  }
}

##</code><code>##

while ($line = <>) {
  # ...
  my $pos = tell ARGV;
  my $next = <>;
  seek ARGV, $pos, 0;
  # ...
}