You may want to see perldoc perlre or at least perldoc perlrequick.

...
my $reg = {
  'open',   qr{<table>},
  'close',  qr{</table>}
};

while( <DATA> ){
    if(m#$reg->{open}# ... m#$reg->{close}#){
       /$reg->{open}|$reg->{close}/ and next;
       # print or whatever ...
    }
}
...
# STDOUT:
# <tr><td>First Table</td</tr>
# <tr><td>Second Table</td</tr>
# <tr><td>Third Table</td</tr>

__DATA__

<table>
<tr><td>First Table</td</tr>
</table>
<table>
<tr><td>Second Table</td</tr>
</table>
<table>
<tr><td>Third Table</td</tr>
</table>
[download]