Without writing it for you, this script shows the general idea on using the search method of the Text widget.

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

my $mw = MainWindow->new( -bg => 'black' );
$mw->geometry('100x30+100+15');

my $file_name = shift || $0;

my $file;
open (FH,"< $file_name");
read( FH,  $file, -s FH );
close FH;

my $search_string = '';
my $kind          = 'exact';

my $stringframe        = $mw->Frame;
my $ss            = $stringframe->LabEntry(
            -label        => 'Search string:',
            -width        => 40,
            -labelPack    => [qw/-side left -anchor w/],
             -textvariable => \$search_string
                 )->pack(qw/-side left/);

my $ss_button = $stringframe->Button( -text => 'Highlight' );
$ss_button->pack(qw/-side left -pady 5 -padx 10/);

my $text = $mw->Scrolled(qw/Text -setgrid true -scrollbars e/);
$text->tagConfigure( 'search',   -foreground => 'red',-background => '
+white' );

$text->insert('0.0', $file);
$text->mark(qw/set insert 0.0/);

my $subframe = $mw->Frame;
my $exact    = $subframe->Radiobutton(
    -text     => 'Exact match',
    -variable => \$kind,
    -value    => 'exact'
);
my $regexp = $subframe->Radiobutton(
    -text     => 'Regular expression',
    -variable => \$kind,
    -value    => 'regexp'
);
$exact->pack( qw/-side left/,   -fill => 'x' );
$regexp->pack( qw/-side right/, -fill => 'x' );

$stringframe->pack(qw/-side top -fill x/);
$subframe->pack(qw/-side top -fill x/);
$text->pack(qw/-expand yes -fill both/);


my $command = sub { &search_text($text,\$search_string,'search',$kind)
+ };
$ss_button->configure( -command => $command );
$ss->bind( '<Return>' => $command );

MainLoop;

######################################################################
+#3

sub search_text {

    # The utility procedure below searches for all instances of a give
+n
    # string in a text widget and applies a given tag to each instance
+ found.
    # Arguments:
    #
    # w -    The window in which to search.  Must be a text widget.
    # string -    Reference to the string to search for.  The search i
+s done
    #           using exact matching only;  no special characters.
    # tag -    Tag to apply to each instance of a matching string.

    my ( $w, $string, $tag, $kind ) = @_;
    #print "@_\n";

    return unless ref($string) && length($$string);

    $w->tagRemove( $tag, qw/0.0 end/ );
    my ( $current, $length ) = ( '1.0', 0 );

    while (1) {
        $current =
          $w->search( -count => \$length, "-$kind", $$string, $current
+, 'end' );
        last if not $current;
        warn "Posn=$current count=$length\n",
          $w->tagAdd( $tag, $current, "$current + $length char" );
        $current = $w->index("$current + $length char");
    }

}    # end search_text
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