each has its own problems in terms of maintainability. Bailing out of the loop early (perhaps an exception was hit, perhaps there was a return or last) will leave the iterator in the middle of the hash. 

> perl -wE '
    sub p {
        my($h) = @_;
        while (my($k, $v) = each %$h) { return "$k=$v" }
    }

    my %h = qw( one 1 two 2 three 3 );
    say p(\%h);
    say p(\%h);
    say p(\%h);
'
three=3
one=1
two=2

[download]

This can lead to some subtle hard to trace bugs. I've been personally bitten by this several times, and I've essentially dropped each from my toolkit; it's simply too easy to leave the iterator in a strange spot.

I also wouldn't be too sure of your optimization suggestion. With a pretty basic benchmark I slapped together (source here) it would appear separate keys/values is faster: 


> ./each-vs-key-values.pl 
BEGIN, hash: short
Benchmark: running each, keys_values for at least 5 CPU seconds...
      each:  6 wallclock secs ( 5.26 usr +  0.01 sys =  5.27 CPU) @ 44
+213.85/s (n=233007)
keys_values:  6 wallclock secs ( 5.63 usr +  0.01 sys =  5.64 CPU) @ 1
+07908.33/s (n=608603)

                Rate        each keys_values
each         44214/s          --        -59%
keys_values 107908/s        144%          --
END, hash: short

BEGIN, hash: long
Benchmark: running each, keys_values for at least 5 CPU seconds...
      each:  7 wallclock secs ( 5.83 usr +  0.00 sys =  5.83 CPU) @ 16
+6.38/s (n=970)
keys_values:  6 wallclock secs ( 5.30 usr +  0.00 sys =  5.30 CPU) @ 6
+95.09/s (n=3684)

             Rate        each keys_values
each        166/s          --        -76%
keys_values 695/s        318%          --
END, hash: long

BEGIN, hash: alphabet
Benchmark: running each, keys_values for at least 5 CPU seconds...
      each:  5 wallclock secs ( 5.24 usr +  0.01 sys =  5.25 CPU) @ 36
+26.29/s (n=19038)
keys_values:  5 wallclock secs ( 5.20 usr +  0.00 sys =  5.20 CPU) @ 1
+3917.12/s (n=72369)

               Rate        each keys_values
each         3626/s          --        -74%
keys_values 13917/s        284%          --
END, hash: alphabet

[download]

This is with a Debian Lenny perl 5.10.1. I'm speculating the overhead of the multiple each calls is killing any gains you get from not iterating a second time; that, or the iteration is cheap because of how the HV is built.

In reply to Re^2: keys and values order on a hash by Somni
in thread keys and values order on a hash by citromatik

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