#!/usr/bin/perl
while($line = <DATA>){
@valid_entities= ('<a>','<td>','<th>','<var>','<br>');
my %htmlenties = map { $_ =>1 } @valid_entities;

$line =~  s/(<(\w+?)(>))/exists $htmlenties{$1} ? $1 : defined ($2) ? "&lt;$2$3" : '&lt;'/eg;

print $line;
}

__DATA__
<Hello>Hi...<BR>how r u<br>