Here's my solution.

#!perl
use warnings;
use strict;
sub rfindi (&@) {
        my $th = $_[0];
        for my $i (1 .. @_ - 1) {
                &$th($_[@_ - $i]) and
                        return @_ - $i - 1;
        }
        return;
}
my @t;
my $e = 0;
while (local $_ = <>) {
        while (m(<(/)?(\w+))g) {
                my $t = uc $2;
                if (!defined($1)) {
                        push @t, $t;
                } else {
                        if ($t eq $t[-1]) {
                                pop @t;
                        } elsif (defined(my $i = rfindi { $_[0] eq $t 
+} @t)) {
                                $e = 1;
                                warn "unclosed ", join(",", @t[$i + 1 
+.. @t - 1]), " tags at ", $t,
" end tag";
                                delete @t[$i .. @t - 1];
                        } else {
                                $e = 1;
                                warn "closing unopened ", $t, " tag";
                        }
                }
                #warn "[$.:$+[0]: @t]\n";
        }
}
@t and do {
        $e = 1;
        warn "unclosed ", join(",", @t), " tags at eof";
};
exit $e;
__END__
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