Removing this would probably help performance a bit as it is totally unnecessary:

 
if(exists($edgeHash{$a})){
    push @{$edgeHash{$a}}, $b;
}else{
    push @{$edgeHash{$a}}, $b;
}
[download]

Unless this is a mistake, either condition of the if test does the same thing.

-----------------------------------
Frank Wiles <frank@wiles.org>
http://frank.wiles.org