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$x = "<tr><td>1</td><td>2</td><td><font>3</font></td><td>4</td><tr>";

print "$x\n";
$x =~ s/<td>.*(<td>.*?)3(.*?<\/td>).*<\/td/<td>$1a$2<\/td>/;
print "$x\n";

<tr><td>1</td><td>2</td><td><font>3</font></td><td>4</td><tr>
<tr><td><td><font>a</font></td></td><tr>