Solution: Either go through the whole hash, or rewind it after using each:

I once worked on a largish codebase that had this as the cause of several bugs in various places. We ended up using variations of things like:

sub foreach_pair (&\%) {
    my ($coderef, $hashref) = @_;
    local ($a, $b);
    keys %$hashref;
    while ( ($a, $b) = each %$hashref ) { $coderef->($a, $b) };
};

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

# if you don't mind remembering what $a and $b are
foreach_pair { print "$a == $b\n" } %foo;

# or if you prefer being explicit
foreach_pair { my ($name, $value) = @_; print "name $name is $value\n"
+ } %foo;
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To avoid the problem and make things a bit more explict.