According to perldoc perlsyn:

If any part of LIST is an array, foreach will get very
confused if you add or remove elements within the loop
body, for example with splice. So don't do that.
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So, it seems to me, you probably don't want to be adding or removing elements while within a for loop. Was that your question?

For example, I just tested this w/ shift. I looped through an array using foreach and did a shift part of the way through:

my @arr = qw/foo bar baz quack/;
for (@arr) {
    print $_, "\n";
    shift @arr if $_ eq "bar";
}
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Here's the output:

foo
bar
quack
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So the shift shifted off "foo", which messed up the internal loop counter, which means that "baz" didn't get displayed. So this probably isn't the best idea.