# $re = exclude(@words);
sub exclude {
  my %words;

  push @{ $words{ quotemeta substr($_, 0, 1) } },
    quotemeta substr($_, 1)
    for @_;

  my $first = "[^@{[ join '', keys %words ]}]*";
  my $rest =
    join "|",
    map "$_(?!" . join("|", @{ $words{$_} }) . ")",
    keys %words;

  return qr/^$first(?:(?:$rest)$first)*$/;
}

my $re = exclude(qw( this that those ));
# print $re;  # for debugging purposes

for ("I like this", "give me that one", "these rock!") {
  print "$_ => " . /$re/;
}