Any reason you are opposed to two lines?

($result = $htmlline)  =~ s/<tag marker>(.+?)</td>/$1/;
$result =~ s/<br(?:\s\/)?>/\n/g;
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Update: If you really want one line: (untested)

( $result = $htmlline ) =~ s/{<tag marker>(.+?)</td>}{join("\n", split
+(/<br(?:\s/?)?>/, $1))}eg;
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