Do not use regular expressions to parse HTML/XML. Assuming your input is indeed HTML, here's a possible solution using Mojo::DOM, based on my code here.

use warnings;
use strict;

print html_filter(<<'END_HTML', qw/pre strong i/), "\n";
aaa<pre>bbb</pre>ccc<www><i>ddd</i><strong>eee</strong>fff</www>ggg
END_HTML

use Mojo::DOM;
sub html_filter {
    my $html = shift;
    my %allowed = map {$_=>1} @_;
    my $walk; $walk = sub {
        my ($in, $out) = @_;
        for my $n ( @{ $in->child_nodes } ) {
            if ( $n->type eq 'cdata' || $n->type eq 'text' )
                { $out->append_content($n->content) }
            elsif ( $n->type eq 'tag' ) {
                if ($allowed{$n->tag}) {
                    my $t = $out->new_tag( $n->tag, %{$n->attr} )
                        ->child_nodes->first;
                    $walk->($n, $t);
                    $out->append_content($t);
                } else { $walk->($n, $out) }
            } # ignore other node types for now
        }
        return $out;
    };
    return $walk->(Mojo::DOM->new($html), Mojo::DOM->new)->to_string;
}

__END__

aaa<pre>bbb</pre>ccc<i>ddd</i><strong>eee</strong>fffggg
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