my $sep = ';'; # field separator
my $expected_field_count = scalar split /$sep/, <>; # header line
while(<>) {
    chomp;
    my @fields = split /$sep/, $_;
    while(@fields < $expected_field_count) {
        my $fieldline = <>;
        my @l = split /$sep/, $_;
        $fields[-1] .= '<br>'. shift @l;
        push(@fields,@l) if @l;
    }
    ...
}