#!/usr/bin/perl

use strict; # https://perlmonks.org/?node_id=11148806
use warnings;

my $text = 'This is just an arbitrary example of text.';

#ONLY TWO WORDS ARE CAPTURED--WE WANT TO HIGHLIGHT ONLY THOSE
my $query = qr~(?:(?:This)|(?:That)).*?(just).*?(arbitrary).*?$~;

#THIS WOULD HIGHLIGHT THE ENTIRE LINE
#$text =~ s~($query)~<span class="highlight">$1</span>~g;

$text =~ $query and do
  {
  for ( reverse 1 .. $#- )
    {
    substr $text, $+[$_], 0, '</span>';
    substr $text, $-[$_], 0, '<span class="highlight">';
    }
  };

print $text, "\n";
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Outputs:

This is <span class="highlight">just</span> an <span class="highlight"
+>arbitrary</span> example of text.
[download]