Hi, let me try to explain with my limited knowledge of perl's internal behaviour ;o)

Your solution converts %target and %source into flat lists, combines it to one list and assigns that list to %target...

Grandfather's solution only inserts the key-value pairs from %source into %target, overwriting already existing pairs.

This is less expensive than converting into lists and reassigning these lists.

I tried a benchmark which shows that Grandfather's solution is faster than yours.

I have to admit, that - until this post - I preferred and used the way you presented (mainly because of readability), but now I have to reconsider ;o)

#!/usr/bin/perl
# vim: set ts=4 sw=4 et sta:
use strict;
use warnings;
use Benchmark qw( cmpthese );

my %target;
my %source;

## prefill
@target{'aa'..'zz' } = 'aa' .. 'zz';
@source{'ww'..'yy', 1..100}     = ( 'ww'..'yy', 1..100 );

cmpthese( -1, {
   grandfather => sub {
        my %t = %target;
        @t{keys %source} = values %source;
    },
   sleepyjay   => sub {
       my %t = %target;
       %t = ( %t, %source );
    },
});

__END__
              Rate   sleepyjay grandfather
sleepyjay    598/s          --        -62%
grandfather 1585/s        165%          --

              Rate   sleepyjay grandfather
sleepyjay    649/s          --        -62%
grandfather 1706/s        163%          --
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