You need to use references.

Try this:

&callsub(\%hashtopass);

sub callsub
{
    my $hashread = $_[0];

    foreach $k (keys $%hashread)
    {
        .
        .
        .
    }
}
[download]

Check this out for more information: http://www.perldoc.com/perl5.6.1/pod/perlref.html