Here is the benchmark test I ran. There seems to be a non-negligible difference, but it isn't big enough to worry about unless you are trying some sort of highspeed realtime application. I didn't do any printouts, to be fair.

#!/usr/bin/perl
use warnings;
use strict;
use Benchmark;

my %bighashname;
my %b;

timethese(10000, {

hashbigname => sub {
     for(1..1000){
       $bighashname{"bighashval$_"} = $_
        }

     for(1..1000){
       $bighashname{"bighashval$_"}++;
        }
},

hashsmallname => sub {
   for(1..1000){
       $b{"b$_"} = $_
        }

     for(1..1000){
        $b{"b$_"}++;
        }
 },

});

#Benchmark: timing 10000 iterations of hashbigname, hashsmallname...
#hashbigname: 39 wallclock secs (39.27 usr +  0.00 sys = #39.27 CPU) @
+ 254.65/s (n=10000)
#hashsmallname: 35 wallclock secs (34.66 usr +  0.00 sys = #34.66 CPU)
+ @ 288.52/s (n=10000)
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