The comma in ,=> is a NOP.

• The fat comma is just a comma (sic).
• multiple "empty" commas are ignored (that's why we have 'undef')

  DB<8> x ["a",=>"b"]
0  ARRAY(0x54e32f0)
   0  'a'
   1  'b'
  DB<9> x ["a",,"b"]
0  ARRAY(0x54e2cd8)
   0  'a'
   1  'b'
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Like choroba already explained it's a syntactic trick to avoid lc to be stringified by =>

edit

so whats happening is a construction of hash pairs, where the keys are normalized to lowercase. The fat-comma is meant to clarify this pair nature.

  DB<12> x map {lc,$_} "A".."C"
0  'a'
1  'A'
2  'b'
3  'B'
4  'c'
5  'C'
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