I don't know if this will help your understanding, but check out this example, it allows you to get tags at the pointerx and pointery positions, thereby limiting the tag search. Hope it helps.

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

my $mw = tkinit;
my $t = $mw->Scrolled('Text',
              -scrollbars => 'osoe'
               )->pack;

for(1..100){
  $t->tagConfigure( 'data'.$_, -data => $_ x 20,
);
}

for(1..100){
 $t->insert('end', 'Line'."$_\n", ['datarider','data'.$_ ]);
} 

$t->tagBind( 'datarider', '<Enter>', sub { getdata($t) } );
$t->tagBind( 'datarider', '<Leave>', sub { getdata($t) } );
$t->bind( '<Motion>', sub{ getdata($t) } );


MainLoop;

sub getdata {
    my ( $text_widget ) = @_;

    my $x = $text_widget->pointerx - $text_widget->rootx;
    my $y = $text_widget->pointery - $text_widget->rooty;
    #print "$x $y\n";
    my $txt_index = $text_widget->index( '@' . $x . ',' . $y );
    #warn $txt_index;
    my ( $line, $char ) = ( $txt_index =~ /^(.+?)\.(.+?)$/ );
    my @tags =  $text_widget->tagNames($txt_index);
    print "@tags\n";

    foreach my $tag(@tags){
       print $text_widget->tagCget($tag,'data'),"\n";
      }

}
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