In Perl, the operator for creating a regexp. Naturally, it's "pick your own delimiters", like all the other q's. If you pick the single quote ' as your delimiter, the insides obey a "single quote" context ($'s and other variable references not expanded). Otherwise, it's "double quote" context, just like for the m// operator. Watch those @'s (and $'s)!

Storing the result of qr// in a variable lets you compile the regexp just once (when the qr// is evaluated). This is easier than the old way of compiling an anonymous sub that does the matching for you.

You may also use the result of qr// inside another regular expression:

$digits = qr/\d+/;
$sign = qr/[+-]?/;
$float = qr/$sign $digits
            (\. $digits)?
            ([Ee]$sign $digits)?/x;