You really do need a parser, so while a pair of substitutions is possible, it's not the most appropriate. 

my %conv = ( '*' => '?', '?' => '#' );
my $conv = quotemeta(join('', keys(%conv)));

my $string = 't?e\\\\xt\\\\* with escapes\\*';

my $result = $string;
for ($result) {
   s/(?<!\\)((?:\\{2})*)([$conv])/$1$conv{$2}/g;
   s/\\(.)/$1/sg;
}

print($result, "\n");

[download]

Your code can be simplified (visually): 

my $string = 't?e\\\\xt\\\\* with escapes\\*';

my $result = '';
for ($string) {
    /\G \\(.) /xgcs && do { $result .= $1;   redo; };
    /\G \*    /xgcs && do { $result .= '?';  redo; };
    /\G \?    /xgcs && do { $result .= '#';  redo; };
    /\G (.)   /xgcs && do { $result .= $1;   redo; };
}

print($result, "\n");

[download]

An optimization of the above: 

my $string = 't?e\\\\xt\\\\* with escapes\\*';

my $result = '';
for ($string) {
    $result .= $1 if /\G ([^\\*?]+) /xgcs;

    /\G \\(.) /xgcs && do { $result .= $1;   redo; };
    /\G \*    /xgcs && do { $result .= '?';  redo; };
    /\G \?    /xgcs && do { $result .= '#';  redo; };
}

print($result, "\n");

[download]

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

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