c:\@Work\Perl\monks>perl -wMstrict -le
"use constant VALID_TAG => qr{ \A [[:alpha:]] [[:alpha:]\d]* \z }xms;
 ;;
 sub html (*@) {
   my ($tag, @strings) = @_;
   ;;
   $tag =~ s{ \A \s+ | \s+ \z}{}xmsg;
   die qq{invalid tag '$tag'} unless $tag =~ VALID_TAG;
   ;;
   return qq{<$tag>@strings</$tag>};
   }
 ;;
 print html(p, qw(how now brown cow));
 my $n = 2;
 print html('h' . $n, 'the rain in spain');
 print html(strike, 'now', html(b, 'is the'), 'time');
 ;;
 print html(42, 'oops...');
"
<p>how now brown cow</p>
<h2>the rain in spain</h2>
<strike>now <b>is the</b> time</strike>
invalid tag '42' at -e line 1.