Could you include a snippet of code that can be run which reproduces the issue and your expected output, it is somewhat unclear what you mean

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);
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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]

Update: added code examples