my %hash = (
    vars => 'vars',
    this => 'this',
    doink => 'doink',
    dink => 'dink',  
);

while(<DATA>) {
    s/\[([^]]+)\]/$hash{$1}/g;
    print;                    
}
 
__END__
<h1>Some beginning HTML up here</h1>

<update>
interpolate [vars] when they
are<p> between brackets like [this].
<p> <a href="[doink]">[dink]</a>
</update>

<small>The ending HTML down here</small>