fancy experimental features? ;-)

#!/usr/bin/perl -l
use strict;
use warnings;

my @str = split /\n/, <<'HTML';
<tr class="meld"><th>Security Code Matched</th><td>foo</td></tr>
<tr><th>Security Code Matched</th><td>N/A</td></tr>
<tr><th class="foo">Security Code Matched</th><td>N/A</td></tr>
<tr><th>Security Code Matched</th><td>10!</td></tr>
<tr><td>Security Code Matched</td><td class="val">N/A</td></tr>
HTML

for (@str) {
    if ( m{
        \bSecurity\ Code\ Matched\b
        (.*?)</tr>
        (?(?{ index($^N, 'N/A') < 0 })!)
        }xsg
    ) {
        print "[[$_]]";
    }
}
[download]

output:

[[<tr><th>Security Code Matched</th><td>N/A</td></tr>]]
[[<tr><th class="foo">Security Code Matched</th><td>N/A</td></tr>]]
[[<tr><td>Security Code Matched</td><td class="val">N/A</td></tr>]]
[download]

i wouldn't use it at home, though ;-)