my @tag_regexes = map { qr/<($_) ?[^>]*>/ } qw/kt bold ital ch\d/;

while (defined (my $line = <DATA>)) {
    die "Invalid element in line #$. ($line). Cannot proceed due to the above error\n"
        unless grep { $line =~ $_ } @tag_regexes;
}

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