#!/usr/bin/perl -w
use strict;

$/=undef;
$_ = <>;

my @ina = /<a(.*?)<\/a/isg;
print "Found unterminated <A> tag containing: '$_'" for grep { /<a/ } @ina;

##</code><code>##

<a href="ok" name="<a> go on">this is ok</a>