&callsub(%hashtopass); 

sub callsub {
   %hashread = @_;

   foreach $k (keys %hashread) {
       print "$k => $hashread{$k}\n";
   }
}

##</code><code>##

&callsub(\%hashtopass, $otherStuff);

sub callsub {
   my ($hashref, $otherParam) = @_; 
# or: my $hashref = shift; if you prefer that

   foreach my $k (keys %{$hashref}) {
       print "$k => $hashref->{$k}\n";
   }
}