Premature optimization is the root of all evil. Just don't do that. Perl will handle allocation good enough most of the times. There may be rare cases where preallocation may be useful, but then, you would use Devel::NYTProf or similar first to find and eliminate other bottlenecks.

To save keystrokes, use my %hash=(); or just my %hash;, both are technically equal. Some people (including me) prefer the first form, refusing to rely on the implementation details of Perl to initialize the variable with an empty hash / empty array / undefined scalar, other people think that the documentation is very clear on this fact and the initializing assignment is just too much and redundant work.

Alexander