I have to admit that it confused me for a long time too.

I thought it's akin to 0+ which enforces scalar context and numeric casting (also in other languages like JS).

But it's basically a No-OP which separates a function from the parens, such that the parens are not interpreted as part of the function call.

It has sufficiently low precedence and doesn't meddle with context:

  DB<9> say ("a".."c")[1]
syntax error at (eval 18)[c:/Strawberry/perl/lib/perl5db.pl:738] line 
+2, near ")["

  DB<10> say (("a".."c")[1])               # how it works
b

  DB<11> say +("a".."c")[1]                # avoid extra parens
b

  DB<12> say +("a".."c")[1..2]             # LIST uneffected
bc

  DB<13>
[download]