$orig='aaa([\+\-]{1})bbb';
$res = '([\\+\\-]{';
$qres = '\\Q$res\\E';

$str=$orig;
$str =~ s!\Q$res\E!!;
print "Replace against variable quoted => $res <=: $str\n";

$str=$orig;
$str =~ s!$qres!!;
print "Replace against variable quoted => $qres <=: $str\n";