Markdown rendering differently

[Andy Fragen]

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I'm using Markdown from the command line using the following syntax.

% echo "string to process" | perl Markdown.pl

If I run % echo "$" | perl Markdown.pl, I get <p>$</p> -- expected

If I run % echo "$8" | perl Markdown.pl, I get nothing. I would expect
<p>$8</p>

If I run % echo "$56" | perl Markdown.pl, I get <p>6</p>, I expect <p>$56</p>

Why is this happening? If I escape the '$' then it renders as expected.
Should I have to escape it?

-- 
Andy Fragen