This is not the case in regex though, $str =~ qq/$ x/; will use the value of $x as a regular expression, while $str =~ qr/$ x/; just doesn't match anything ($ is the position before \n, so the characther after that can neither be a space nor x). This means whitespace is not ignored when doing so yields a non-sensical expression :P

perl -E "$x = 'Hello'; say $ x =~ qr/$ x/ ? 'Match': 'No match'"
No match

perl -E "$x = 'Hello'; say $ x =~ qq/$ x/ ? 'Match': 'No match'"
Match
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Edit: it's whitespace, not blankspace, got the wrong word :)