I can say that it's not a good idea. Look at what you get when you do this:

my @foo = qw/a b c d e f g/;
for my $x (@foo) {
    push @foo, 'd' if $x eq 'd';
    print $x;
}
[download]

The iterator keeps marching through the d's, adding new ones to the end. A better way to do it might be:

push @foo, map {$_ eq 'd' ? 'd' : ()} @foo;
print for @foo;
[download]

I doubt that it's implmentation-dependent. The iterator is stepping through the array, and by the time it gets to the end of the array, there's another element there to step through.

Still, it's not a nice trick to play. Here's an even more diabolical one:

for (@foo) {
  print;
  shift(@foo) if $_ eq 'c';
}
print "\n@foo\n";
[download]