I started wondering how effcient my solution might be since Perl is not as efficient with hashes as it is arrays. So I compared it to what tilly posted.

@$old{keys %$new} = values %$new;
I did a benchmark on each block of code.

Benchmark: timing 1000000 iterations of tilly_way, trs80_way...
 tilly_way: 11 wallclock secs 
    (10.27 usr +  0.00 sys = 10.27 CPU) @ 97408.92/s (n=1000000)
 trs80_way: 26 wallclock secs 
    (24.16 usr +  0.00 sys = 24.16 CPU) @ 41382.16/s (n=1000000)

So don't do it the trs80_way for two reasons. One it is hard for someone new to Perl to know what is going on in the trs80_way. Second it is slower, although it won't make a hill of beans difference if you only work with small hashes.

That said, I still think the trs80_way looks cooler :^)


Reference data:
Benchmark script:

use Benchmark;

$hash1 = { a => '56',
           b => '75',
           c => '87'
         };
         
$hash2 = { d => '45',
           f => '46',
           g => '67',
          };
          

timethese(1000000, { 
            trs80_way => sub { $hash1 = { %$hash1, %$hash2 };  },
            tilly_way => sub { @$hash1{keys %$hash2} = values %$hash2;
+ },
        }
        );
[download]

Benchmark module documentation (this is a little out of date)

I vaguly remember this in either TPJ or the Perl Cookbook, anyone else remember?