Here's why each is not a good idea:

Suppose you were to use each? Each call to each remembers its position within a hash. Only the first call would actually check to see if the hash was empty. The second and later calls would only be checking to see if there was a next element. So if statement #2 would return false if empty or if there were only one element. If statement #3 would return false there were 0,1, or 2 elements.

But the problems run even deeper. Any function inside the conditional block will miss the first key-value pair if it tries to use a while each loop . Bugs like this are hard to track down because the confused while loop may not be in your own code - it could be in a third party subroutine! Code like this:

use strict;
use warnings;
sub printAll;

my %h=(a=>1, b=>2, c=>3);

my @aKeys = keys %h;
print "full set of keys: <@aKeys>\n";
print "print all key-value pairs:\n";
printAll(\%h) if (each %h);

# subroutine using while each loop
# possibly buried somewhere deep in a third party module
sub printAll {
  my $h=shift;
  while (my ($k, $v) = each(%$h)) {
    print "$k, $v\n";
  }
}
[download]

outputs

full set of keys: <c a b>
print all key-value pairs:
a, 1
b, 2

#whoops - no c, 3 printed out!!!
[download]

Best, beth