#!/usr/bin/perl
while($line = <DATA>){

@valid_entities= ('<a>','<abbr>','<acronym>');
my %htmlenties = map { $_ =>1 } @valid_entities;

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

print $line;
}

__DATA__
<helloe>How r u <a> www.google.com</a>
<Hi>How r u </hi>