I've been trying all sorts of ideas to supress the \\ being escaped, but I'm stuck.

my $test = <<'TEXT';
illegal\\characters\*example?
TEXT
my @illegal = qw( \  * ? );
my @legal   = qw( bs a q );
my $c = 0;
for my $val ( @illegal ) {
    $test =~ s/\Q$val\E/[$legal[$c]]/g;
    $c++;
    }
print "$test\n";
[download]

This produces:

illegal[bs][bs]characters[bs][a]example[q]