man, you are hard to please

use strict;
use warnings;

my $text="<tr><td>1</td><td>2</td><td>qw<font>3</font></td><td>4</td><
+tr>";

$text =~ s/<td>((?:(?!<td>).)*)3((?:(?!<\/td>).)*)/$1a$2/;
print "$text";
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outputs

<tr><td>1</td><td>2</td>qw<font>a</font></td><td>4</td><tr>
[download]