Perl Gotcha II

Don't exit an each loop unless you really know what you do.
Consider the following code

  1 my %h = ( one => 1, two => 2, three => 3 );
  2 sub find_key {
  3   my $val = shift;
  4   while ( my ($k,$v) = each %h ) {
  5     return $k if $v == $val;
  6   }
  7 }
  8 find_key(2) eq 'two' or die "key not found";
  9 # and again
 10 find_key(2) eq 'two' or die "key not found";

The call to find_key in line 10 fails, because it does not iterate through %h again, but continues where the last each call left off.

This is the documented behavior, but one might not expect it.
perldoc says:

The next call to "each" after that will start iterating again.  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".