#!/usr/bin/perl -w
use strict;
use Tk;
my $top = MainWindow->new;

my $t = $top->Text();
$t->tagConfigure("boring",    -foreground => "blue");
$t->tagConfigure("special",   -foreground => "red");

# Change the match rule according to taste
my @words = qw(your be is silly);
my $regex = join("|", @words);
$regex = qr/(.*?)\b($regex)\b/s;

my $text = "If you want to be immune from silly letters, don't carry your monomark in your hat.\n";

while ($text =~ /$regex/gc) {
    $t->insert("end", $1, "boring");
    $t->insert("end", $2, "special");
}
$t->insert("end", $1, "boring") if $text =~ /(.+)/gs;

$t->pack;
MainLoop;