The answer is in perlsub.

$_[0] becomes an alias to the sub's parameter. In this case, that param is $item->{'title'}, so the change ripples back as you've experienced.

Do it this way:

sub strip_tags{
    my $string = shift;  # Or my $string = $_[0];
    $string =~ s/<[^>]+>//gs;
    return $string;
}
[download]

With that method you're making a copy of the string and working on it.