From @zoffixznet

The parse error occurs with alphabetic infix between the s/// but seems to be fine with a symbol-based infix​:

<Zoffix> m​: say(s/^// and s/^//)
<camelia> rakudo-moar 405519​: OUTPUT«===SORRY!=== Error while compiling <tmp>␤Unable to parse expression in argument list; couldn't find final ')' ␤at <tmp>​:1␤------> say(s/^//⏏ and s/^//)␤ expecting any of​:␤ infix␤ infix stopper␤»
<Zoffix> m​: say(s/^// andthen s/^//)
<camelia> rakudo-moar 405519​: OUTPUT«===SORRY!=== Error while compiling <tmp>␤Unable to parse expression in argument list; couldn't find final ')' ␤at <tmp>​:1␤------> say(s/^//⏏ andthen s/^//)␤ expecting any of​:␤ infix␤ infix stopper␤»
<Zoffix> m​: say(s/^// or s/^//)
<camelia> rakudo-moar 405519​: OUTPUT«===SORRY!=== Error while compiling <tmp>␤Unable to parse expression in argument list; couldn't find final ')' ␤at <tmp>​:1␤------> say(s/^//⏏ or s/^//)␤ expecting any of​:␤ infix␤ infix stopper␤»
<Zoffix> m​: say(S/^// or S/^//)
<camelia> rakudo-moar 405519​: OUTPUT«===SORRY!=== Error while compiling <tmp>␤Unable to parse expression in argument list; couldn't find final ')' ␤at <tmp>​:1␤------> say(S/^//⏏ or S/^//)␤ expecting any of​:␤ infix␤ infix stopper␤»

These are parsed fine​:
<Zoffix> m​: say(s/^// && s/^//)
<camelia> rakudo-moar 405519​: OUTPUT«Method 'match' not found for invocant of class 'Any'␤ in block <unit> at <tmp> line 1␤␤»
<Zoffix> m​: say(S/^// && S/^//)
<camelia> rakudo-moar 405519​: OUTPUT«Method 'match' not found for invocant of class 'Any'␤ in block <unit> at <tmp> line 1␤␤»

From @zoffixznet

The s/// appears not to be involved ( http://irclog.perlgeek.de/perl6-dev/2016-07-10#i_12816423 )​:

<lucasb> m​: sub f {}; f(1 or 2)
<camelia> rakudo-moar 89470a​: OUTPUT«===SORRY!=== Error while compiling <tmp>␤Unable to parse expression in argument list; couldn't find final ')' ␤at <tmp>​:1␤------> sub f {}; f(1⏏ or 2)␤ expecting any of​:␤ infix␤ infix stopper␤»

From @zoffixznet

Not a bug as per http://irclog.perlgeek.de/perl6-dev/2016-07-10#i_12816463

The or is lower precedence and the invocation parentheses aren't grouping parentheses so the `or` ends up or'ing the unclosed `sub-call(`

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