#!/usr/bin/perl

use strict;
use warnings;

my $string = qq|<a href="/cgi-programming-with-perl.zip">cgi-programming-with-perl.zip</a>|;
my $item = 'perl';

$string =~ s/(\s?[-\w\.]*(?<!\/)$item[-\w\.]*\s?)/<b>$1<\/b>/gi;
print $string . "\n";

output :
<a href="/<b>book-cgi-programming-with-perl.zip</b>"><b>book-cgi-programming-with-perl.zip</b></a>