It can be done (at least approximated) in a somewhat more traditional s///g manner, but I'd use two substitutions to deal first with metacharacters, then unescaping. 
my $escape       = qr/(?<!\\)\\/;
my $meta_char    = qr/(?<!$escape)[?*]/;

no warnings 'qw';
my %meta = qw(
    ? #
    * ?
);

my $result = my $string = 't?e\\\\xt\\\\* with e\\scapes\\*';
$result =~ s/($meta_char)/$meta{ $1}/g;
$result =~ s/$escape(.)/$1/gs or die;

print "$string\n";
print "$result\n";

[download]
It transforms the example string in the same way as your code. I wouldn't swear that it does for all input strings.

Anno

In reply to Re: Replace only unescaped metachars by Anno
in thread Replace only unescaped metachars by Anonymous Monk

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