Mail::Address Under Taint

I'm having a problem running MIME::Lite which uses Mail::Address under
tainting.  The following program illustrates that there is a problem in
Mail::Address:

#!/usr/bin/perl -T

use Mail::Address;

my $addy = '[email protected]';
my ($o) = Mail::Address->parse($addy);

{
  my ($a,$b,$c)=@{$o};
  my $arg = $b;
  my $nada=substr($arg,0,0);
  local $@;
  eval { eval "# $nada" };
  if (length($@) != 0) {
    warn "'$arg' IS TAINTED";
  }
  else {
    warn "'$arg' IS CLEAN";
  }
}

This outputs the following:

'[email protected]' IS TAINTED at ./test.pl line 15.

I've isolated this down to sub _tokenise(), near line 116 of Mail::Address.
Before the line that reads:

      s/^("([^"\\]|\\.)*")\s*//       # "..."
   || s/^(\[([^\]\\]|\\.)*\])\s*//    # [...]
   || s/^([^\s\Q()<>\@,;:\\".[]\E]+)\s*//
   || s/^([\Q()<>\@,;:\\".[]\E])\s*//
     and do { push(@words, $1); next; };

$_ is untainted.  After this line $1 is tainted.  I'm not exactly clear on
what is going on here, or why it would taint $1.  Perhaps you could shed
some
light on the subject.

-- Jeff Horn