$s = 'fred';;
print 'fred' =~ m[\b\Q$s\E\b] ? 'matched' : 'no match';;
matched

$s = '$fred';;
print '$fred' =~ m[\b\Q$s\E\b] ? 'matched' : 'no match';;
no match

$s = '$fred';;
print '$fred' =~ m[^\Q$s\E$] ? 'matched' : 'no match';;
matched