If getMem() is returning a hash reference, then you are dereferencing the hashref into a hash with %{ ... } and then dereferencing a second time using The Arrow Operator, thus the warning. The preferred syntax would be:

sub get_memory_tot {
        return $WAI->kBToMB($WAI->getMem()->{'MemTotal'} );
}

sub get_memory_free {
        return $WAI->kBToMB( $WAI->getMem()->{'MemFree'} );
}

sub get_swap {
        return $WAI->kBToMB( $WAI->getMem()->{'SwapTotal'});
}
[download]

See perlref and/or perlreftut for more explanation.