Read (and heed) what the others above said about premature optimization. But here's what I get for one pathalogical exaple using large hash keys. 
greg@sparky:~/perl$ cat hashing_short 
#!/usr/bin/perl

my $short="a";
my $long="This is a long hash key"x1000;
my %h;

$h{$short}++ for (1..1000000);


greg@sparky:~/perl$ time hashing_short 

real    0m0.376s
user    0m0.375s
sys     0m0.001s

greg@sparky:~/perl$ cat hashing_long
#!/usr/bin/perl

my $short="a";
my $long="This is a long hash key"x1000;
my %h;

$h{$long}++ for (1..1000000);


greg@sparky:~/perl$ time hashing_long  

real    2m46.893s
user    2m38.196s
sys     0m0.278s

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In reply to Pathological example by sleepingsquirrel
in thread code execution speed and variable name length by zentara

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