my $allowed = qr{kt$|bold$|ital$|ch};
while (defined (my $line = <DATA>)) {
    die "Element $1 invalid,line #$.\n"
        if (grep { $_ !~ /^$allowed/ } $line =~  /<([^>].+?)>/g);
}

__DATA__
<kt>
<bold>
<ital>
<ch1>
<ch_named>
Line of content
this line has an unspecified <ch> tag 

<ch><Bold>