Always think twice before using each. If you exit the loop early the iterator will not be reset if you enter the loop again:

my %hash = qw/ one 1 two 2 three 3 four 4 five 5 /;

print "pass 1-----\n"; once_thru();
print "pass 2-----\n"; once_thru();

sub once_thru {
    while ( my ( $k, $v ) = each %hash ) {
        last if $k eq 'five';
        print "$k $v\n";
    }
}
__END__
pass 1-----
three 3
pass 2-----
one 1
two 2
four 4
[download]

From each:

There is a single iterator for each hash, shared by all "each", "keys", and "values" function calls in the program; it can be reset by reading all the elements from the hash, or by evaluating "keys HASH" or "values HASH".

The pitfalls could be made much clearer, IMHO.