I won't reiterate haukex's excellent points. Instead, it's worth a mention that if you are just stripping out all characters which match (or don't match) a given set then you do not need the heavy duty regex engine at all.

use strict;
use warnings;

use Test::More tests => 1;

my $tag  = "A quick-brown fox or 3? %&)( \\\n\n";
my $want = 'a quick-brown fox or 3  ';

$tag = lc $tag;
$tag =~ y/a-z0-9 _+.-//cd;

is $tag, $want;
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Add more tests of course to make sure you cover all bases. See y/// in perldoc for more.