while (<DATA>)
{
    s/\\n/\n/g;
    print "yep\n"  if /$PRE($VALID1+)$PST/;
}
[download]

Thanks, that does make sense, but does "break" the data driven approach I am trying to come up with...I have expanded the example and maybe someone will come up with a different idea...if not I'll do it the way suggested ;-)

$PRE    = '\[\s*(';
$VALID1 = '[-a-zA-Z0-9_.* \t\n]';
$VALID2 = '[-a-z0-9_.*\n]';
$VALID3 = '[a-zA-Z]';
$VALID4 = '[-a-zA-Z0-9]';
$PST    = ')\s*\]';
while (<DATA>)
{
    s/\\n/\n/g; #Are these harmless if
    s/\\t/\t/g; #not needed???
    print "yep\n"  if m/$PRE($VALID1+)$PST\s*
                        $PRE($VALID2+)$PST\s*
                        $PRE($VALID3+)$PST\s*
                        $PRE($VALID4+)$PST\s*
                      /x;
}


__DATA__
[TEST \n DATA] [ TEST DATA ]  [ 2345423 ] [ TEST\tDATA ]\n
[TEST \n DATA] [ TEST DATA ]  [ 2345423 ] [ TEST DATA ]\n
[TEST \n DATA] [ TEST DATA ]  [ 2345423 ] [ TEST\tDATA ]\n
[download]

$PRE    = '\[\s*';
$VALID1 = '[-a-zA-Z0-9_.* \t\n]';
$VALID2 = '[-a-z0-9_.*\n]';
$VALID3 = '[a-zA-Z]';
$VALID4 = '[-a-zA-Z0-9]';
$PST    = '\s*\]';
while (<DATA>)
{
    s/\\n/\n/g; #Are these harmless if
    s/\\t/\t/g; #not needed???
    print "yep\n"  if m/$PRE($VALID1+)$PST
                        $PRE($VALID2+)$PST
                        $PRE($VALID3+)$PST
                        $PRE($VALID4+)$PST
                      /ox;
}


__DATA__
[TEST \n DATA] [ TEST DATA ]  [ 2345423 ] [ TEST DATA ]\n
[TEST \n DATA] [ TEST DATA ]  [ 2345423 ] [ TEST\tDATA ]\n
[TEST \n DATA] [ TEST DATA ]  [ 2345423 ] [ TEST DATA ]\n
[download]

You didn't say which (if any) of the three data records is supposed to yield "yep"... it looks like none of them will, because $VALID3 specifies letters only, and all three data lines have only digits in the third field. Also, for any of them to match, $PST should include "\s*" after the close bracket, as well as before it (or maybe this should be added before the open bracket in $PRE).

You do have the right notion for converting a literal (two character) '\n' or '\t' into the corresponding regex for the given type of whitespace.

Note that some portions of your regexes can be simplified:  [a-zA-Z0-9_] is really just "\w", and if you want to match space, newline and tab, you might as well just use "\s".

Are $VALID1 and $VALID2 really supposed to accept periods and asterisks, as well as alphanumerics and whitespace? (Just checking... sometimes people tend to make the mistake of putting ".*" inside of square brackets when they really have something else in mind.)

my $re = qr/\[\s*([-a-zA-Z0-9_.*\s]+)\s*\]/;
my ($last);
while (<DATA>)
{
    chomp;
    $_ = $last.$_ if($last);
    if (/$re/){
        print "yep\n";
        $last = '';
    }elsif(substr($_, -1, 1) ne ']'){
        $last = $_;
    }else{
        $last = '';
    }
}

__DATA__
[TEST 
DATA]
[ TEST DATA ]
[download]

/o must go (look around for many threads about the /o bug ...)