You must be careful, if input comes from a TEXTAREA, your text will contain a CRLF combination for every end-of-line. You're better off replacing those with plain newlines, "\n", before you insert them in a database, for example. Or before feeding it back into the HTML for the next rendered page.

s/\x0D\x0A/\n/g
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And don't forget to HTML-escape your string, when inserting it into the HTML, both for the TEXTAREA for edit and in the plain HTML for view, the former before you insert your <BR> tags — or you might end up escaping your freshly inserted tags, too.

Oh, and it'd still be nice if you kept the newlines. For example:

s/$/<BR>/mg;
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or

s/\n/<BR>\n/g;
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