$orig='aaa([\+\-]{1})bbb';
$qre="\\Q([\\+\\-]{\\E\\d\\Q})\\E";
$bre='\(\[\\\\\+\\\-\]\{\d\}\)';

$str=$orig;
$str =~ s!\Q([\+\-]{\E\d\Q})\E!!;
print "Replace against literal quoted: $str\n";

$str=$orig;
$str =~ s!\(\[\\\+\\\-\]\{\d\}\)!!;
print "Replace against literal bslashed: $str\n";

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

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