How about a regexp?
I usally do this by making something like this:
$line =~ s/<<VARIABLE>>/$variable/;

with $line being a line from the HTML file I'm currently parsing...

In your case it should be something like this:

if ($line ne /\<.+\>/) {
 $line =~ s/<<VARIABLE>>/$variable/; 
}
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I'm not defect!