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.

In reply to Re: Disambuating $ in (especially /m) regexps by Anonymous Monk
in thread Disambuating $ in (especially /m) regexps by tlhackque

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