In Regexes§

See primary documentation in context for Conjunction: &&

Matches successfully if all &&-delimited segments match the same substring of the target string. The segments are evaluated left to right.

This can be useful for augmenting an existing regex. For example if you have a regex quoted that matches a quoted string, then / <quoted> && <-[x]>* / matches a quoted string that does not contain the character x.

Note that you cannot easily obtain the same behavior with a lookahead, that is, a regex doesn't consume characters, because a lookahead doesn't stop looking when the quoted string stops matching.

say 'abc' ~~ / <?before a> && . /;    # OUTPUT: «Nil␤» 
say 'abc' ~~ / <?before a> . && . /;  # OUTPUT: «｢a｣␤» 
say 'abc' ~~ / <?before a> . /;       # OUTPUT: «｢a｣␤» 
say 'abc' ~~ / <?before a> .. /;      # OUTPUT: «｢ab｣␤» 

Just like with ||, empty first branches are ignored.