"perldoc -f each" gives you the explanation:

When called in list context, returns a 2-element list consisting of the key and value for the next element of a hash, so that you can iterate over it. When called in scalar context, returns only the key for the next element in the hash.

(hmmm ... http://perldoc.perl.org appears to still be down)

For example:

#!/usr/local/bin/perl

use strict;
use warnings;

my %couples = (
   george  => 'gracie',
   abbot   => 'costello',
   johnson => 'boswell',
);

my( $key, $val );

# scalar context - return keys one at a time
$key = each %couples;
print "$key : $couples{$key}\n";

$key = each %couples;
print "$key : $couples{$key}\n";

$key = each %couples;
print "$key : $couples{$key}\n";

# reset each's internal iterator
$key = each %couples;

# list context
while ( ($key, $val) = each %couples ) {
  print "$key - $val\n";
}
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