When you say you passed a hash, do you mean you called the routine with a hash as a parameter? and if so did you call it like
my $value=mySub(%hash); or my $value=mySub(\%hash;)
Example call below

#! /usr/bin/perl
use strict;
use warnings;
my %hash=qw(fruit apple dairy cheese cereal oats);
sub whatis{
   my $hash=shift;
   for my $category (keys %{$hash}){
      print "$hash->{$category} is a $category\n";
   }
}
whatis(\%hash);
[download]

Or do you mean that the subroutine returns a hash and you wish to access a value of this hash by it's key? Horrible example below

#! /usr/bin/perl
use strict;
use warnings;
sub foodCategories{
   my %hash=qw(fruit apple dairy cheese cereal oats);
   return \%hash;
}
sub whatis{
   for my $category (keys %{foodCategories()}){
      print &foodCategories->{$category}," is a $category\n";
   }
} 
whatis();
[download]

1. Appears (to me) to be asking how to convert a sub name to the corresponding code ref .oO(is that the only value of a subroutine ?).
2. IMO, the 2nd bears no relation to the 1st [question]

In short, I suggest you read How do I post a question effectively? ...closely.

We could probably give you detailed answers if you provided the code you tested with along with an explanation of what you wanted it to do and how the actual results differed from your intended results. Without that information, though, we'd just be guessing at what you want.