duckyd's solution checks that all lines have at least one valid tag in them. I think you were looking for a check that each tag, when present, is a valid tag, skipping over any lines that don't have tags.

Also, if there's more than one tag on a line, you probably want to check all of them. You can get all the tags to check easily with the "g" option to the m// match operator, if you use it in an array context. Note that the match has to be non-greedy, using "+?" instead of "+", otherwise it will lump tags toghether.

Try this, which catches the second tag in the last line. (tested)

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>
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