can someone explain why scoping works this way here?

That question implies that the scoping works differently elsewhere -- it doesn't. you get that error regardless of the for loop...

laptop:~> perl -Mstrict -e 'my $i=1, my $j=$i+1; print $i + $j;'
Global symbol "$i" requires explicit package name at -e line 1.
Execution of -e aborted due to compilation errors.
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...the behavior of the comma oporator is not changed by being used in a for loop.