1. first, you declare the variable 2. then, you pass it as Hash to the function that reads it .. 3. then, again deal with same variable in the usage()... Is there a way I have to deal with the variable just once??

Maybe foobar(\my %hash); or foobar(my $href = {...}); can get you part of the way there.

I only glanced at them, but you might make use of Getopt::ArgvFile (specifying files that are automatically read?) or Getopt::Lucid (which talks about reading in config file in its synopsis).