my @tag_regexes = ( qr/^kt$/,
                    qr/^bold$/,
                    qr/^ital$/,
                    qr/^ch\d$/,
                  );

while (defined (my $line = <DATA>)) {
    $line =~ /<([^ >]+)/;
    my $tag = $1;
    die "Invalid element $tag in $.. Cannot proceed due to the above error\n"
        unless grep { $tag =~ $_ } @tag_regexes;
    print "tag: $tag\n";
}


__DATA__
<kt>
<bold>
<ital>
<ch1>
<kt someattribute="somevalue">
<bold someattribute="somevalue">
<ital someattribute="somevalue">
<ch1 someattribute="somevalue">