The question then is is this violation of the general prohibition safe or was I lucky

Now I am confused. What do you expect,

1. Perl loops over the original array, whatever you do to the array in the meantime
2. Perl loops over the modified array

Frankly, I have no idea what to expect, you can make a case for both behaviors. In such a case, I'd say: don't do it. What will happen in this case might change over time, possibly because someone decided to fix perl's "erratic behavior".

What I think is safe, is this:

1. Loop over the original array: make a temporary copy.

   foreach(my @temp = @arr) {
       push @arr, 'd' if $_ eq 'a';
       push @arr, 'e' if $_ eq 'b';
       push @arr, 'f' if $_ eq 'c';
       print $_;
   }
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2. if you want to loop over the modified array, use the loop index:

   for(my $i = 0; $i < @arr; $i++) {
       local $_ = $arr[$i];  
       push @arr, 'd' if $_ eq 'a';
       push @arr, 'e' if $_ eq 'b';
       push @arr, 'f' if $_ eq 'c';
       print $_;
   }
   [download]

p.s. I was expecting this to work:

foreach(() = @arr) {
   print;
}
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but it doesn't loop even once. I think that's a bit weird, since

$x = () = @arr;
[download]

actually sets $x to the size of the array.