[perl #130045]

[Samantha McVey]

Actually it has nothing to with <$a>, and this triggers it as well:
m: ' ' ~~ m:s/ /;
OUTPUT«===SORRY!=== Error while compiling <tmp>␤Null regex not allowed␤at 
<tmp>:1␤------> ' ' ~~ m:s/ ⏏/;␤»

If this is indeed a bug, this should probably be renamed.