More of an HTML question, but just have it print the value between the textarea tags .. Note i removed the line breaks -- if you leave them there they will be part of the default value.

<textarea rows="4" name="[%element.name %]" >[% element.default %]</te
+xtarea>
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As for the second part, it seems like you want to add the '$' or whatever delimiter to element.default before you pass it to the template -- may sometihng like $element->{default} =~ s/\n/\$/sg;