As per perlop:

Interpolation in patterns has several quirks: $|, $(, $), "@+" and "@-" are not interpolated, and constructs $var[SOMETHING] are voted (by several different estimators) to be either an array element or $var followed by an RE alternative.

It's in "Gory details of parsing quoted constructs". Therefore, $|, $( and $) are not variables in re patterns. Also note that

If "'" (single quote) is the delimiter, no interpolation is performed on the PATTERN.