You're talking about your Findnext button? Use

$f->Button(-text => "FindSelectionNext", -command => [$t->{'rtext'} =>
+ "FindSelectionNext"])
[download]

http://search.cpan.org/grep?cpanid=NI-S&release=Tk-804.028_501&string=Findnext&i=1&n=1&C=10

http://cpansearch.perl.org/src/NI-S/Tk-804.027/Text/Text.pm

######################################################################
+##
# get current selected text and search for the next occurrence
sub FindSelectionNext
{
 my ($w) = @_;
 my $selected;
 eval {$selected = $w->SelectionGet(-selection => "PRIMARY"); };
 return if($@);
 return unless (defined($selected) and length($selected));

 $w->FindNext('-forward', '-exact', '-case', $selected);
}
[download]