[exikyut]

Hmm.

  perl -le '$A = "ABCDEF"; substr($A, 3, 0) = 123; print $A'
  ABC123DEF

[imglorp]

Look again at that comment. He wasn't remarking about a substr, it was the curious syntax where the LHS of an assignment was actually an argument to the RHS function.

[aduitsis]

https://perldoc.perl.org/perlsub.html#Lvalue-subroutines