#!/usr/bin/perl

use strict;
use warnings;

undef $/;

$_ = <DATA>;

my %repl = qw /
    <      &lt;
    >      &gt;
    foo    bar
    bar    foo
/;

s/(<\s*br\s*>|[<>]|foo|bar)/$repl{$1}||$1/ge;

print;

__DATA__
<br> Don't do the regex
< br > Don't do the regex
<  br> Don't do the regex
<br > Don't do the regex

<this> Do the regex
<is> Do the regex
<a> Do the regex
<test> Do the regex
<this is a test> Do the regex




<br> Don't do the regex
< br > Don't do the regex
<  br> Don't do the regex
<br > Don't do the regex

&lt;this&gt; Do the regex
&lt;is&gt; Do the regex
&lt;a&gt; Do the regex
&lt;test&gt; Do the regex
&lt;this is a test&gt; Do the regex