my $remains = $string;
my %parsed = ();
while ( $remains =~ /^\s*\<(.*)$/ ) { # for each tag...
    $remains = $1;
    #we expect an identifier
    ( $remains =~ /^\s*(\w+)(.*)$/ ) or die 'expected identifier';
    my $kwd = $1;
    $remains = $2;

    if ( $kwd eq 'vertical' ) {
        ( $remains =~ /^\s*(\w+)\s*\=\s*(\w+)(.*)$/ or die 'expected assignment';
        $parsed{ $kwd } and die 'duplicate definition';
        $parsed{ $kwd }{ $1 } = $2;
        $remains = $2;
        Exhausted( \$remains );
        next; # next tag
    }

    if ... # next tag processor goes here

}