Build dynamic regexes. Maybe something like:

my $regex = 's/(\w+)\s+tacos/$1 yummy tacos/g';
$regex =~ s/(\\s\+)(\w)/$1(<\\w\\w>)$2/g;       # Handle first part of
+ substitution
my $match = $2;
$regex =~ s/ $match/\$2$match/g;                # Handle second part o
+f substitution
eval "$regex";
[download]

That would handle the transform from the first to the second. Ideally, you would re-evaluate your regexes and build them using some regex builder. The builder would handle the optional tags and making sure they stayed in after the substitutions.