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

my $string =<<'EOF';
<tr>
<td>aaa</td>
<td>aaa</td>
</tr>
<tr>
<td>NOTWANTED</td>
</tr>
<tr>
<td>bbb</td>
<td>bbb</td>
</tr>
EOF

# try to remove all table rows with NOTWANTED
$string =~ s/<tr.+?NOTWANTED.+?<\/tr>//gsm;

print $string;